R1:8a0dfa94d438

Make configured and EditEngine defaults work correctly for custom fields

Summary:
Ref T10004. Fixes T5158. There was a long-standing issue with defaults not working properly, but EditEngine has made it more obvious because it's a lot easier to set defaults now.

The issue is basically that the defaults are getting set as the field's real value early on, so when we go to generate the transaction "old value" later, we build a transaction that uses the //new// value as both the "new value" and "old value".