Create system accounts dynamically from DB

[rfc2822]

We may store accounts in the database and then create system accounts dynamically (e.g. at sync) from the database information. This yields the following benefits:

• Managed DAVx⁵ Auto-Provisioning (bitfireAT/davx5#120): because accounts are automatically created from the database, the auto-provisioning process only has to detect the services and put them into the database
• "Samsung update bug" (bitfireAT/davx5#176): in rare cases, Samsung/maybe other devices "forget" all system accounts (or user data stored in the accounts) – wouldn't be a problem anymore when system accounts are dynamically created
• Android Backup (bitfireAT/davx5#132): dynamically created system accounts, would allow us to only back up the database, i.e. the default "full backup" would back up everything, without further work.
• AccountManager doesn't reliably store user data / AccountSettings. Sometimes they are not written correctly or get lost for some reason.

Possible steps

Accounts:

• Add a new accounts table with (auto) migration – Create Accounts table in DB #934
• Add logic to dynamically create accounts from info in accounts table
  • Calendar/tasks sync: manage CalDAV accounts before sync
  • Contacts sync: manage CardDAV accounts before sync
  • No need for "main accounts" anymore – CardDAV accounts can be used without CalDAV account

Settings:

• Store AccountSettings in database instead of AccountManager – Store AccountSettings in DB #935

Depends on #934
Depends on #935

[github-actions]

This PR/issue depends on:

• Create Accounts table in DB #934
• Store AccountSettings in DB #935