Feature: Better dav admin

[tandy-1000]

Is your feature request related to a problem? Please describe.
Monica currently uses sabredav to provide cal/card dav, for those who are self hosting, they may already be using a dav server, such as Nextcloud or Baikal. Since Monica doesn't really allow administration of its dav server, this can lead to redundancy. I don't really want to host 2 dav servers if possible.

This issue is somewhat related to #2738, but I have made a separate issue.

Describe the solution you'd like
Either:

• allow disabling of the built-in dav server to use other existing service,
• or, expand the dav administration functionality to allow adding new address books / calendars
  • This would also aid further features, like using different calendars / address books for different groups such as 'Home' and 'Work'.

Describe alternatives you've considered (optional)
If this is out of the project scope, I could simply keep two dav servers...