Create Accounts table in DB

[rfc2822]

Create an Accounts table in DB, which will become the SSOT of the account list in the future.

Context: Currently the AccountManager is used to store the list of accounts and associated data. When the account is lost in the AccountManager, it's also not available in DAVx5. Instead, DAVx5 should contain all information about accounts and be able to create them on demand in the future.

The plan is to

1. create/update/delete Accounts in the DB whenever a DAVx5 system account is created/updated/deleted,
2. have a migration that copies DAVx5 system accounts into the Accounts table.

So we have then an up-to-date Accounts table in the DB, but all logic still uses the system accounts. (Scope of this issue).

Then we can gradually move the DB, until we only use the DB and dynamically manage the system accounts. (Scope of future issues)

Depends on #936
Depends on #878

[github-actions]

This PR/issue depends on:

• Centralize access to accounts in AccountRepository #936
• Remove concept of main accounts #878