Convert hooks to OOP hooks with attributes

Should probably kill _group_relation_type_manager() now, but that requires a CR.

Should go over each hook manually to see if it still works.

Follow-ups:

• Deprecate _group_relation_type_manager() (v2/3) and remove in v4.
• Add aliases to our services so they can be autowired (v2/3/4)

Pushed a commit that uses hook ordering and the new theme initial preprocess callbacks. The entire module file is gone now and I've bumped minimum required version to 11.2. That last bit may make tests fail as we don't have an official 11.2 release yet.

Tests are failing on me forgetting to convert tokens.inc and views.inc, also need to tackle submodules still.