R1:1a1da7d6c276

Integrate auxiliary fields into Maniphest transactions

Summary:
- When changing auxiliary field values, use transactions.
- Clean up some of the load/save logic for auxiliary fields so it's a little
more performant.

NOTE: The transaction display of auxiliary fields is incredibly hacky, I'll
follow up with a more nuanced approach but wanted to limit scope here.

Test Plan: Created and edited tasks with custom fields configured; created and
edited tasks without custom fields configured.

Reviewers…