feat: classify emails by importance based on subjects

[st3iny]

Partly addresses #3968

Closes #8257

Part 5: The great rebasing ...

Improvements

• Classify emails by importance using a new and improved classification pipeline
• Persist classifier models (and transformer pipelines) in memory cache only
• Drop the oc_mail_classifiers table

How to tests?

1. Have some important mails in your inbox.
2. Make sure to have the classification enabled: Account settings modal -> "Data collection consent" ✅ and "Mark as important" ✅
3. Train a model: occ mail:account:train -vvv <account-id> (extract account id with occ mail:account:export <user-id>)
4. Receive some mails and observe that some are marked as important and some aren't.