When modifying models that are in the database, we sometimes need to

stash and restore references (constraints). The code that did this would

walk through all fields on all models, looking for anything that pointed

to the model, and would then execute SQL to temporarily remove them or

to introspect them.
This was happening even for models that weren't yet added to the

database, just present in the model registry.
These are now skipped. Only models that are known to be installed now

have their refs stashed.

Encountered this in an upgrade from Review Board 4 -> 5. Verified that

this solved it.
Work is still needed to add unit tests here, but that will come later,

so as not to block current work.