help: Add "How can I help?" section to Contribute page

This doesn't use the how-can-i-help package, but it is a similar
implementation that fetches the same data from udd.debian.org.

There are sections for the following issues:
- Packages that will be removed from Debian testing
- Packages that are not in Debian testing
- Good first issues for beginners
- Issues for which the package maintainer has requested help

The overall "How can I help?" section is collapsed by default.

Test:
- View the page in stable and testing container.
- Run help functional tests.

Closes: #536

Signed-off-by: James Valleroy <jvalleroy@mailbox.org>
[sunil: Minor indentation fix]
Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Sunil Mohan Adapa <sunil@medhas.org>

plinth/modules/help/templates/help_contribute.html

@@ -42,4 +42,76 @@
       {% trans 'Learn more...' %}
     </a>
   </p>
+
+  <h3>{% trans "How can I help?" %}</h3>
+  <p>
+    {% blocktrans trimmed %}
+      Below is a list of opportunities for contributing to Debian. It
+      has been filtered to only show packages that are installed on
+      this system.
+    {% endblocktrans %}
+  </p>
+  <p>
+    <a class="btn btn-default collapsed collapsible-button" role="button"
+       data-toggle="collapse" href="#collapse-issues" aria-expanded="false"
+       aria-controls="collapse-issues">
+      <span class="fa fa-chevron-right fa-fw" aria-hidden="true"></span>
+      {% trans "Show issues" %}
+    </a>
+
+    <div class="collapse" id="collapse-issues">
+      <h4>{% trans "Packages that will be removed from Debian testing" %}</h4>
+      <ul>
+        {% for issue in testing_autorm %}
+          <li>
+            {% for package in issue.packages %}
+              <b>{{ package }}</b> {% endfor %}
+            ({% trans "source package:" %}
+            <a href="https://tracker.debian.org/pkg/{{ issue.source }}">
+              {{ issue.source }}</a>):
+            {% for bug in issue.bugs %}
+              <a href="https://bugs.debian.org/{{ bug }}">
+                #{{ bug }}</a>
+            {% endfor %}
+          </li>
+        {% endfor %}
+      </ul>
+
+      <h4>{% trans "Packages that are not in Debian testing" %}</h4>
+      <ul>
+        {% for issue in no_testing %}
+          <li>
+            <b>{{ issue.package }}</b>
+            ({% trans "source package:" %}
+            <a href="https://tracker.debian.org/pkg/{{ issue.source }}">
+              {{ issue.source }}</a>)
+          </li>
+        {% endfor %}
+      </ul>
+
+      <h4>{% trans "Good first issues for beginners" %}</h4>
+      <ul>
+        {% for issue in gift %}
+          <li>
+            <b>{{ issue.package }}</b>:
+            <a href="https://bugs.debian.org/{{ issue.bug }}">
+              #{{ issue.bug }}</a>
+            {{ issue.title }}
+          </li>
+        {% endfor %}
+      </ul>
+
+      <h4>{% trans "Issues for which the package maintainer has requested help" %}</h4>
+      <ul>
+        {% for issue in help %}
+          <li>
+            <b>{{ issue.package }}</b>:
+            <a href="https://bugs.debian.org/{{ issue.bug }}">
+              #{{ issue.bug }}</a>
+            {{ issue.title }}
+          </li>
+        {% endfor %}
+      </ul>
+    </div>
+  </p>
 {% endblock %}

plinth/modules/help/views.py

@@ -3,16 +3,20 @@
 Help app for FreedomBox.
 """
 
+import gzip
+import json
 import mimetypes
 import os
 import pathlib
 
+import apt
 from django.core.files.base import File
 from django.http import Http404, HttpResponse, HttpResponseRedirect
 from django.template.response import TemplateResponse
 from django.urls import reverse
 from django.utils.translation import get_language_from_request
 from django.utils.translation import gettext as _
+import requests
 
 from plinth import __version__, actions, cfg
 from plinth.modules.upgrades.views import (get_os_release,
@@ -27,8 +31,52 @@ def index(request):
 
 def contribute(request):
     """Serve the contribute page"""
-    return TemplateResponse(request, 'help_contribute.html',
-                            {'title': _('Contribute')})
+    response = requests.get('https://udd.debian.org/how-can-i-help.json.gz')
+    data = gzip.decompress(response.content)
+    issues = json.loads(data)
+
+    # Split issues according to type and filter for installed packages.
+    testing_autorm = []
+    no_testing = []
+    gift = []
+    help_needed = []
+    cache = apt.Cache()
+    for issue in issues:
+        if issue['type'] == 'testing-autorm':
+            for package in issue['packages']:
+                try:
+                    if cache[package].is_installed:
+                        testing_autorm.append(issue)
+                        break
+                except KeyError:
+                    pass
+        elif issue['type'] == 'no-testing':
+            try:
+                if cache[issue['package']].is_installed:
+                    no_testing.append(issue)
+            except KeyError:
+                pass
+        elif issue['type'] == 'gift':
+            try:
+                if cache[issue['package']].is_installed:
+                    gift.append(issue)
+            except KeyError:
+                pass
+        elif issue['type'] == 'help':
+            try:
+                if cache[issue['package']].is_installed:
+                    help_needed.append(issue)
+            except KeyError:
+                pass
+
+    return TemplateResponse(
+        request, 'help_contribute.html', {
+            'title': _('Contribute'),
+            'testing_autorm': testing_autorm,
+            'no_testing': no_testing,
+            'gift': gift,
+            'help': help_needed,
+        })
 
 
 def feedback(request):