When associating a model manager, the type remains the default of a

Manager[_ModelT]. Unless given a more specific type, callers won't see

the methods on these managers.
This change goes through the codebase and adds explicit typing for each

model manager association. As a note for future associations, these must

be typed as ClassVars in order to safely override the original types.

Unit tests pass.
Verified that I could access methods for some of the ones relevant to my

query work without the type checker complaining.