feat: display info message if app is not deployed

[pradel]

Currently blocked by dokku/dokku#4134

[pradel]

We can now continue with this one dokku/dokku#4134 (comment).
The best would be:

• when we sync, check if the app is deployed
• save it in the db as a boolean flag
• on the page along with the message add a button to update the app status

[josegonzalez]

This should be unblocked. Also, exit codes are now slightly different (or should be) if an app does not exist. See https://dokku.github.io/release/dokku-0.24.0 for details, but basically if you get an exit code of 20, that means the app does not exist. Might save on ssh commands :)