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.

{F115918}

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