Make Phriction diff-of-changes view draft-aware and clarify some language

Summary:
Ref T13077. Ref T13197. See PHI840.

• In the "History > Diff/Compare" view, the button language wasn't draft-aware.
• Revise language to avoid the word "Revert", since this can be ambiguous.
  • "Edit this page, starting with an older version of the text" is now "Edit Older|Current|Draft Version X".
  • "Mark this older version of the page as the current published version" is now "Publish Older Version".
• Let the user edit the current published version, too, since this is a reasonable operation if there are drafts.

Test Plan: Navigated the history diff view, saw better button and action text.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13197, T13077

Differential Revision: https://secure.phabricator.com/D19668