R1:9d59086d0148

Consolidate transaction generation in EditType objects

Summary:
Ref T9132. This is a bit more cleanup to make adding CustomField support easier.

Right now, both `EditField` and `EditType` can actually generate a transaction. This doesn't matter too much in practice today, but gets a little more complicated a couple of diffs from now with CustomField stuff.

Instead, always use `EditType` to generate the transaction. In the future, this should give us less total code and make more things work cleanly by…