Migrate `MyUser`s to `drift`

[SleepySquash]

Part of #27

Synopsis

Hive is obsolete.

Solution

This PR further migrate to drift by:

• moving MyUser persistence to drift;
• implementing CommonDatabase and ScopedDatabase concept for multiple MyUser accounts.

Checklist

• Created PR:
  • [x] In draft mode
  • [x] Name contains issue reference
  • [x] Has type and k:: labels applied
• Before review:
  • [x] Documentation is updated (if required)
  • [x] Tests are updated (if required)
  • [x] Changes conform code style
  • [x] CHANGELOG entry is added (if required)
  • [x] FCM (final commit message) is posted or updated
  • [x] Draft mode is removed
• [x] Review is completed and changes are approved
  • [x] FCM (final commit message) is approved
• Before merge:
  • [x] Milestone is set
  • [x] PR's name and description are correct and up-to-date
  • [x] All temporary labels are removed

[SleepySquash]

FCM

Migrate `MyUser`s persistence from Hive to `drift` (#1016, #27)

- impl `MyUsers` table and conversion methods for it
- refactor `MyUser` and `Auth` stores to use `MyUserDriftProvider`

Additionally:
- split `AppDatabase` into `CommonDatabase` and `ScopedDatabase`