Model refactor, part 3

[DingDongSoLong4]

This is the last part of my model refactoring, mainly focusing on repositories:

• 67725fb removes manager.Repository in favour of just models.Repository, and some cleanup around uses of txn.WithTxn and unnecessary Repository or txn.Manager function arguments or struct fields.
• ea432c7 adds models.TxnManager to the various other Repository structs, and adds NewRepository functions to easily create those repositories from a models.Repository. Also more cleanup around usage of those repositories.
• 2bff764 adds a mock Database struct for the entire database, which makes it easier to create and use the mock ReaderWriters.
• d3bf97e adds an AssertExpectations utility method to the mock Database, which just calls AssertExpectations on all the ReaderWriters.
• 40f37d9 adds a "parent" routes struct, adding a utility withReadTxn method for use with a request context.
• 2941448 moves the default movie image from models to internal/static, and adds exported convenience functions to internal/static which panic if reading fails, since this should never happen.
• And then e261ecb is just a minor refactor of the default performer image boxes in internal/api/images.go.