Ref T3886. Fixes the removed TODO. This also implements the generally reasonable policy "you have to be able to see an object in order to see its transactions". That was implicit before (we never load transactions without loading an object first) but is now explicit.
This fixes bad (nonspecialized) rendering of custom field transactions in Projects, and shortly in Differential, where stories would read "alincoln edited this object." instead of a more specific string.