Improve rendering of "default value changed" custom form transactions to at least have all the information

Summary:
Ref T13319. Currently, transactions about changes to a default form value use a raw internal key for the affected field and don't show the actual value change.

An ideal implementation will likely require us to specialize a great deal of rendering, but we can do much better than we currently do without too much work:

• Try to pull the actual EditField object for the key so we can getLabel() it and get a human-readable label (like Visible To instead of policy.view).
• Add a "(Show Changes)" action that dumps the raw values as more-or-less JSON, so you can at least figure out what happened if you're sophisticated enough.

Test Plan:
Before:

{F6516640}

After:

{F6516642}

The quality of "Show Details" varies a lot. For some fields, like "Description", it's pretty good:

{F6516645}

For others, like "Assigned To", it's better than nothing but pretty technical:

{F6516647}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13319

Differential Revision: https://secure.phabricator.com/D20594