feat: revise migrations

[WitoDelnat]

• Remove init containers. MongoDB connection and migrations are now awaited within the cloud-api main function.
• Remove split between MongoDB migrations and indexing. This was causing problems where indexing happened before migrations. You cannot index a time series database before it's defined (and that happens in a migration).
• Migrations now happen during Sync instead of a Pre/Post hook. These hooks were either too early (database does not exist yet) or too late (new code expects migrations to have happened or behaviour is unexepcted).

I originally had this PR, but I decided to start over to simplify changes necessary.