Add local drafts backend

[miaulalala]

Fixes https://github.com/nextcloud/mail/issues/7076

Move the draft handling from IMAP to our local messages mailbox.

To do:

• [ ] Provide API endpoints
• [ ] Tests
• [ ] Background Job that syncs drafts without UID to IMAP
• [ ] Background Job that syncs drafts from IMAP to the local mailbox

There was some discussion about whether the drafts box should be unified or not.

• [x] ~~Clarify whether we need to split messages on a per account basis (this should be possible to handle with a few changes)~~ Neither. Local drafts are not visible to the user ever but have an account ID assigned.