Import Apps in Automatic Update Mode

This is a larger MR consisting of two commits, one for the backend and one for the frontend.

Adding an app from a remote repository now causes this app to automatically track changes in the remote app. So if something in the remote app changes, it will also change in the local app. That is especially convenient for new app versions.

The user is prevented from editing the local app for as long as the app tracks the remote. There is the option to disable this app tracking in the UI, so manual edits will be possible.

Closes #15, #56