We took a bunch of constants out of model classes and put them into model/constants.py. But outside the context of the models, some of the constants are in classes that don't make sense (e.g. a bunch of image constants in Identifier). And some model classes have plenty of constants that weren't moved.

This ticket tracks the work necessary to consolidate all nontrivial model-related constants into model/constants.py and put them into classes that make sense.