Ref T13559. Substantially correct the client logic for "Save" and "Cancel" actions to handle unusual cases.

Quoting behavior:

• Quoted a comment.
• Cancelled the quoted comment without modifying anything.
• Reloaded page.
  • Before changes: quoted comment still exists.
  • After changes: quoted comment is deleted.
• Looked at comment count in header, saw consistent behavior (before: weird behavior).

Empty suggestion behavior:

• Created a new comment on a suggestable file.
• Clicked "Suggest Edit" to enable suggestions.
• Without making any text or suggestion changes, clicked "Save".
  • Before changes: comment saves, but is empty.
  • After changes: comment deletes itself without undo.

General behavior:

• Created and saved an empty comment (deletes itself).
• Created and saved a nonempty comment (saves as draft).
• Created and saved an empty comment with an edit suggestion (saves).
• Created and saved an empty comment with a suggestion to entirely delete lines -- that is, no suggestion text (saves).
• Edited a comment, saved without changes (save).
• Edited a comment, save deleting all text (saves -- note that this is intentionally without undo, since this is a lot of steps to do by accident).
• Cancel editing an unchanged comment (cancels without undo).
• Cancel editing a changed comment (cancels with undo).
  • Undo'd, got text back.
• Cancel new comment with no text (deletes without undo).
• Cancel new comment with text (deletes with undo).
  • Undo'd, got text back.
• Saved a quoted comment with no changes (saves -- note that this is intentionally not a "delete", since just quoting someone seems fine if you click "Save" -- maybe you want to come back to it later).