Allow CustomField to provide ApplicationTransaction change details

Summary:
Ref T3886. Ref T418. For fields like "Summary" and "Test Plan" where changes can't be summarized in one line, allow CustomField to provide a "(Show Details)" link and render a diff.

Also consolidate some of the existing copy/paste, and simplify this featuer slightly now that we've move to dialogs.

Test Plan:
{F115918}

• Viewed "description"-style field changes in phlux, pholio, legalpad, maniphest, differential, ponder (questions), ponder (answers), and repositories.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3886, T418

Differential Revision: https://secure.phabricator.com/D8284