R1:533e799c5f7e

Modernize task/revision edges and write inverse transactions

Summary:
Ref T5245. See some discussion in D9838.

When we attach object A to object B, we'd like to write transactions on both sides but only write the actual edges once.

To do this, allow edge types to `shouldWriteInverseTransactions()`. When an edge type opts into this, have editors apply the inverse transactions before writing the edge. These inverse transactions don't actually apply effects, they just show up in the transaction log.

Test…