Replace databaseInterface with Django models

~~`databaseInterface` is still imported in a few places in MCPServer, primarily for the `getUTCTime` function that will be moved elsewhere, and for database locking that may be removeable.~~

See comments below.
Inlined both `sip_id` and used `SIPUUID` directly instead of `unitidentifier`. This also lets me avoid using kwargs dicts in favour of directly passing kwargs, so that's nice.