Cleanup static-resource table and create avatar table in DB

[speigg]

We should have a dedicated avatar table, with id, name, modelResourceId, and thumbnailResourceId columns.

The static-resource table needs to be normalized; it should only contain these keys: id, url, mimeType, createdAt, updatedAt, statusCode. Everything else should be moved into separate tables (if needed).

[barankyle]

Avatars should have display name (non unique) and slugified identifier, which is UUID of avatar plus slugified name. slugified identifer is used to name files.