backups: Allow selecting a single app from URL when creating backup

Take app_id in a URL fragment and fill that as the default selected app in
create backup form. This URL can be used in apps to create a backup link.

Tests:

- Visit /plinth/sys/backups/create/bepasty/. Only bepasty app will be selected.

- Visit /plinth/sys/backups/create/foo/. No apps are selected.

- Visit /plinth/sys/backups/create/. All apps are selected.

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

plinth/modules/backups/forms.py

@@ -110,6 +110,7 @@ class CreateArchiveForm(forms.Form):
         components = api.get_all_components_for_backup()
         choices = _get_app_choices(components)
         self.fields['selected_apps'].choices = choices
+        if not self.initial or 'selected_apps' not in self.initial:
             self.fields['selected_apps'].initial = [
                 choice[0] for choice in choices
             ]

plinth/modules/backups/urls.py

@@ -15,8 +15,8 @@ urlpatterns = [
     re_path(r'^sys/backups/$', BackupsView.as_view(), name='index'),
     re_path(r'^sys/backups/(?P<uuid>[^/]+)/schedule/$', ScheduleView.as_view(),
             name='schedule'),
-    re_path(r'^sys/backups/create/$', CreateArchiveView.as_view(),
-            name='create'),
+    re_path(r'^sys/backups/create/(?:(?P<app_id>[1-9a-z\-_]+)/)?$',
+            CreateArchiveView.as_view(), name='create'),
     re_path(r'^sys/backups/(?P<uuid>[^/]+)/download/(?P<name>[^/]+)/$',
             DownloadArchiveView.as_view(), name='download'),
     re_path(r'^sys/backups/(?P<uuid>[^/]+)/delete/(?P<name>[^/]+)/$',

plinth/modules/backups/views.py

@@ -116,6 +116,14 @@ class CreateArchiveView(SuccessMessageMixin, FormView):
         context['title'] = _('Create a new backup')
         return context
 
+    def get_initial(self):
+        """Return initialization arguments to the form."""
+        initial = super().get_initial()
+        if 'app_id' in self.kwargs:
+            initial['selected_apps'] = [self.kwargs['app_id']]
+
+        return initial
+
     def form_valid(self, form):
         """Create the archive on valid form submission."""
         repository = get_instance(form.cleaned_data['repository'])