backups: Disable create archive when no supported apps are installed

Signed-off-by: James Valleroy <jvalleroy@mailbox.org>
Reviewed-by: Joseph Nuthalapati <njoseph@thoughtworks.com>

plinth/modules/backups/templates/backups.html

@@ -39,6 +39,7 @@
     <p>{{ paragraph|safe }}</p>
   {% endfor %}
 
+  {% if available_apps %}
   <p>
     <a title="{% trans 'Create archive' %}"
        role="button" class="btn btn-primary"
@@ -46,6 +47,15 @@
       {% trans 'Create archive' %}
     </a>
   </p>
+  {% else %}
+  <p>
+    {% blocktrans trimmed %}
+      No apps that support backup are currently installed. After any app
+      that supports backup has been installed, then it will be possible to
+      create backups.
+    {% endblocktrans %}
+  </p>
+  {% endif %}
 
   <h3>{% trans 'Backup archives' %}</h3>
   {% if not archives %}

plinth/modules/backups/views.py

@@ -29,6 +29,7 @@ from django.views.generic import FormView, TemplateView
 from urllib.parse import unquote
 
 from plinth.modules import backups
+from .backups import _list_of_all_apps_for_backup
 
 from .forms import CreateArchiveForm, ExportArchiveForm
 
@@ -45,6 +46,8 @@ class IndexView(TemplateView):
         context['info'] = backups.get_info()
         context['archives'] = backups.list_archives()
         context['exports'] = backups.get_export_files()
+        apps = _list_of_all_apps_for_backup()
+        context['available_apps'] = [x[0] for x in apps]
         return context