email: views: Implement tab rendering

plinth/modules/email_server/templates/email_server.html

@@ -0,0 +1,13 @@
+{% extends "app.html" %}
+
+{% load i18n %}
+
+{% block configuration %}
+  {{ tabs|safe }}
+  <p>
+    <a href="/rspamd/">
+      {% trans "Visit Rspamd administration interface" %}
+    </a>
+  </p>
+  {{ block.super }}
+{% endblock %}

plinth/modules/email_server/views.py

@@ -1,15 +1,56 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
-import plinth
+import io
+
+import plinth.views
+from django.shortcuts import render
+from django.utils.html import escape
+from django.utils.translation import ugettext_lazy as _
+
 from . import forms
 
+tabs = [
+    ('', 'Home'),
+    ('alias', 'Alias'),
+    ('relay', 'Relay'),
+    ('security', 'Security')
+]
+
 
 class EmailServerView(plinth.views.AppView):
     """Server configuration page"""
     app_id = 'email_server'
     form_class = forms.EmailServerForm
+    template_name = 'email_server.html'
 
     def form_valid(self, form):
         # old_settings = form.initial
         # new_status = form.cleaned_data
         # plinth.actions.superuser_run('email_server', ['--help'])
         return super().form_valid(form)
+
+    def get_context_data(self, *args, **kwargs):
+        context = super().get_context_data(*args, **kwargs)
+        context['tabs'] = render_tabs(self.request)
+        return context
+
+
+def render_tabs(request):
+    sb = io.StringIO()
+    sb.write('<ul class="nav nav-tabs">')
+    for page_name, link_text in tabs:
+        if request.path.endswith('/' + page_name):
+            cls = 'active'
+        else:
+            cls = ''
+        if cls == 'active':
+            href = '#'
+        else:
+            href = escape('./' + page_name)
+
+        sb.write('<li class="nav-item">')
+        sb.write('<a class="nav-link {cls}" href="{href}">{text}</a>'.format(
+            cls=cls, href=href, text=escape(_(link_text))
+        ))
+        sb.write('</li>')
+    sb.write('</ul>')
+    return sb.getvalue()