Defer reading responses columns until first use.

The previous implementation read them at class load time. This would
fail if the model was loaded when the database was empty. The
development-mode eager model loading added in 85f98a87 caused all the
models to be loaded during db:reset in development.
Correct annotation corrupted in dd740c46f.
Ignore nested models for annotation.

None of the nested models are actually AR models, so this change
a) avoids a bunch of noise as the annotator gets confused, and 
b) does not add bogus annotations to similarly-named models.

This commit also removes the bogus annotations from the nested models.
Filter associations by MdesRecord-ness. #3170.

This is what we actually are trying to find here — which associated
values can be converted to MDES records?