views: Don't send app to template context

- Improves template security as methods of the app object can't be called from
within templates.

Tests performed:

- Visit a few app pages.

- For each of the following template files, ensure that 'app' variable is not
used:

  - app.html

  - All templates that extend app.html

  - Templates included from the above templates.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Reviewed-by: Veiko Aasa <veiko17@disroot.org>

plinth/views.py

@@ -190,7 +190,6 @@ class AppView(FormView):
         """Add service to the context data."""
         context = super().get_context_data(*args, **kwargs)
         context.update(self._get_common_status())
-        context['app'] = self.app  # XXX: Remove this for template security
         context['app_id'] = self.app.app_id
         context['is_running'] = app_is_running(self.app)
         context['app_info'] = self.app.info