Always handle autosave setting when saving SettingsModel (fixes #1092)

[phihos]

Fixes #1092 - This PR attempts to fix the autostart setting not being set correctly when importing. I deliberately chose not to duplicate the code from the misc tab to the init_db function. Instead I tried to keep it DRY and use signals from Pewee. This has the advantage that is does not matter where the SettingsModel is saved - the autostart is always set up afterwards.
This is only a suggestion how to fix it and I know that this is a bit different from an architectural point of view. We basically pull logic from a view to the models. I am always open to a different approach.

[m3nu]

Looks fine. Maybe we will find other uses for Peewee Signals.

https://docs.peewee-orm.com/en/latest/peewee/playhouse.html#connecting-handlers