Reworked notifications, notify for events

[aapeliv]

Backend checklist

• Formatted my code by running autoflake -r -i --remove-all-unused-imports src && isort . && black . in app/backend
• Added tests for any new code or added a regression test if fixing a bug
• All tests pass
• Run the backend locally and it works
• Added migrations if there are any database changes, rebased onto develop if necessary for linear migration history

[aapeliv]

This only applies to those with new notifications enabled, so it should be fine to merge.

[aapeliv]

These are the defaults:

[aapeliv]

Critical parts left:

• test for event notifs
• adjust digest frequency/disable it