ReferenceDynamic - Save lots of irrelevant queries when finding backreferences

[colemanw]

Overview

Follows up on 834f598 to fix a long-standing FIXME in the dynamic reference code.

Before

Checking for dynamic backreferences searched every other entity in the database, not just relevant ones, with a fixme about it in the code.

After

Fixme fixed.

Technical Details

When this code was first written most dynamic references had an open-ended entity_table target column which could theoretically join to anything. Over the years we've been adding whitelists to those columns since most of these entities were only intended for a handful of possible targets. So at the time of writing this fixme, there was no reliable way to use a whitelist to limit the scope of reference searches across the database. Now, for most entities, there is.

A few are still missing whitelists so this grandfathers them in with the old behavior. For all the rest, finding references should be faster and make more sense.

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[eileenmcnaughton]

whoops - looks like this PR is causing test fails - one of the functions it calls got deprecated since the tests passed against it