Struct validation

[tsmethurst]

This PR adds validation tags for all database model structs, and adds a struct validator in internal/validate, as well as tests for all of the defined structs.

It also reworks some of the structure of the packages to make a bit more sense, and adds a stub for Bun migration.

gtsmodel is now the only place that contains structs that enter the database, and it only contains structs that enter the database. Reason for this: when we do migrations, we can basically copy-paste whatever the current state of gtsmodel is into a subfolder of migrations that pertains to the specific migration ID, so each migration has a copy of the gtsmodels it pertains to, as they were at the time the migration was created. This will make it easier to play back or forward from a specific migration.

Note that this test doesn't actually hook the validation into the database logic yet, it only adds struct tags and validation tests to prepare for creating these db hooks in future

Interesting stuff in the PR starts from here downwards:
https://github.com/superseriousbusiness/gotosocial/pull/186/files#diff-7c8fa43ec6d65ce61a4f0617935c0e2fae26f679ffb4ac43ac06f41437521fd3

Tests are here onwards: https://github.com/superseriousbusiness/gotosocial/pull/186/files#diff-52727fb181f9f77e0dc72951e43903d0fc5355ed355f71923fb91d2c60518c16

[NyaaaWhatsUpDoc]

Just ran this PR with SQLite and confirmed all tests are passing. Aside from mentioned issues, looks good!