help: Link to updates page when new version is available

Closes: #1332.

Signed-off-by: Fioddor Superconcentrado <fioddor@gmail.com>
[sunil: Simplify the message for link to upgrade page]
[sunil: Use url method instead of hard-coded path]
[sunil: Cosmetic changes]
Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Sunil Mohan Adapa <sunil@medhas.org>

plinth/modules/help/templates/help_about.html

@@ -65,8 +65,10 @@
     {% endblocktrans %}
 
     {% if new_version %}
+      {% url 'upgrades:index' as upgrades_url %}
       {% blocktrans trimmed %}
-        There is a new {{ box_name }} version available.
+        There is a new {{ box_name }} version
+        <a href="{{ upgrades_url }}">available</a>.
       {% endblocktrans %}
     {% else %}
       {% blocktrans trimmed %}

plinth/modules/help/views.py

@@ -7,7 +7,6 @@ import mimetypes
 import os
 import pathlib
 
-from apt.cache import Cache
 from django.core.files.base import File
 from django.http import Http404, HttpResponse, HttpResponseRedirect
 from django.template.response import TemplateResponse
@@ -16,7 +15,8 @@ from django.utils.translation import get_language_from_request
 from django.utils.translation import ugettext as _
 
 from plinth import __version__, actions, cfg
-from plinth.modules.upgrades.views import get_os_release
+from plinth.modules.upgrades.views import (get_os_release,
+                                           is_newer_version_available)
 
 
 def index(request):
@@ -45,12 +45,10 @@ def support(request):
 
 def about(request):
     """Serve the about page"""
-    cache = Cache()
-    freedombox = cache['freedombox']
     context = {
         'title': _('About {box_name}').format(box_name=_(cfg.box_name)),
         'version': __version__,
-        'new_version': not freedombox.candidate.is_installed,
+        'new_version': is_newer_version_available(),
         'os_release': get_os_release()
     }
     return TemplateResponse(request, 'help_about.html', context)

plinth/modules/upgrades/views.py

@@ -31,8 +31,6 @@ class UpgradesConfigurationView(AppView):
         return {'auto_upgrades_enabled': upgrades.is_enabled()}
 
     def get_context_data(self, *args, **kwargs):
-        cache = Cache()
-        freedombox = cache['freedombox']
         context = super().get_context_data(*args, **kwargs)
         context['can_activate_backports'] = upgrades.can_activate_backports()
         context['is_backports_requested'] = upgrades.is_backports_requested()
@@ -40,7 +38,7 @@ class UpgradesConfigurationView(AppView):
         context['log'] = get_log()
         context['refresh_page_sec'] = 3 if context['is_busy'] else None
         context['version'] = __version__
-        context['new_version'] = not freedombox.candidate.is_installed
+        context['new_version'] = is_newer_version_available()
         context['os_release'] = get_os_release()
         return context
 
@@ -73,11 +71,15 @@ class UpgradesConfigurationView(AppView):
         return super().form_valid(form)
 
 
-def get_os_release():
-    """Returns the Debian release number and name
+def is_newer_version_available():
+    """Returns whether a newer Freedombox version is available."""
+    cache = Cache()
+    freedombox = cache['freedombox']
+    return not freedombox.candidate.is_installed
 
-    Note: The Help module calls this function also.
-    """
+
+def get_os_release():
+    """Returns the Debian release number and name."""
     output = 'Error: Cannot read PRETTY_NAME in /etc/os-release.'
     with open('/etc/os-release', 'r') as release_file:
         for line in release_file: