Use the flask-native global configuration store instead of rolling our own

[matrss]

Every Flask app has a .config attribute to store configuration. Additionally, for MSColab we have mscolab_settings. I think this is redundant, we could just use app.config for everything that should be made globally available in the application in a key-value fashion.

This would also become important once there is not just a singular global APP instance, but the possibility to instantiate more then one Flask app, as each app needs its own configuration. This is something that mscolab_settings is not set up for ATM.

This came up in #2499 (comment).

[ReimarBauer]

we used mostly the same pattern as for mswms_settings. Also there we maybe can move the server configurations to the app.config.