Add 'On Maintenance' view in the different apps

There has been many errors recently that breaks the process of scheduling a meeting. We should have a way to prevent users to use the tool if there is a major issue causing problems, like a `On Maintenance` message.

We should also let the admins to change the value of whether the app is available or not trough the admin app.

Start by locating the different apps involved in scheduling meetings and the admin app that controls availability; no specific files or tests are named in the issue. Done means users are prevented from using the tool during a major issue, see an “On Maintenance” message, and admins can change the app availability setting.

Written by the indexing model from the issue text.