Add accounts to DB

[rfc2822]

Purpose

Previously, only system accounts (of type DAVx5 account) were used for everything account-related. However we had services in the database, which referenced to system accounts.

This PR adds accounts to the database to advance into a direction that allows us to have a reliable SSOT for accounts that is under our control (system accounts aren't).

In the scope of phase 1 (this PR), accounts will additionally be mirrored in the database, but system accounts remain the actual data source. In a future PR, this will be reversed: the database then becomes the actual data source and system accounts can be managed as required.

Short description

• Make AccountRepository the true SSOT for accounts (includes getting Accounts always over fromAccount(name)).
• Create/update/delete accounts in the database when they're modified.

TODO:

• Check whether account_type_test is still neccesary → see https://github.com/bitfireAT/davx5-ose/tree/test-account-type
  • If yes, do we need the account type in the accounts table? Where else?
  • If no, remove
• accountRepository.fromName(service.accountName) not necessary when accountName is a foreign key

Depends on #1224

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

[github-actions]

This PR/issue depends on:

• Remove test account type #1224