Remove concept of main accounts

[rfc2822]

Since the sync is being performed with workers instead of the Sync Adapter Framework, the concept of a "main account" for address book accounts has become obsolete.

We still need address book accounts for storing address books, but they are no longer required to be tied to a "main" account and could from now on exist on their own.

We should remove the concept of a "main" account as much as possible and maybe deprecate it where too hard to remove right now.

• We can find the "main" account for an address book in it's corresponding DB collection (via serviceId).
• We can find all (or specific) address books of a "main" account via the DB carddav collections as well. The URL is the common property between a DB carddav collection and a LocalAddressBook collection. Note that the association should in the future be made via ID.

---

Depends on #875
Depends on bitfireAT/davx5#603

[github-actions]

This PR/issue depends on:

• Provide database Collection to SyncManager #875

[sunkup]

This is harder than I anticipated, because LocalAddressBook extends AndroidAddressBook which is tied to an account (of course) and so has a field called "account" already. The name "mainAccount" is quite descriptive ...

[rfc2822]

and so has a field called "account" already. The name "mainAccount" is quite descriptive ...

Yes, but we shouldn't need it anymore, but use the collection ID as identifier instead. So

DB: collection with ID (belongs to service and account, but that shall not matter in this context anymore) ↔ LocalAddressbook with ID (association via ID) ↔ address book account (not related to main account anymore, but to the ID instead)