RollbackAction: Implement AJAX interface and require POST

RollbackAction: Implement AJAX interface and require POST

Similar to WatchAction (converted in commit 77cdf1919).

* Make FormAction::getFormFields not abstract.
  In most cases this will just be an empty array.

* Convert RollbackAction from FormlessAction to FormAction and implement the
  required error handling scenarios (mostly moved out of from the old method, or
  duplicated from the WikiPage method where necessary).

* In most cases the in-between form is never used since a JavaScript handler
  takes over the link and uses the API over AJAX instead. In the no-js fallback
  (as well as for any existing tokenless rollback links) copy the GET parameters
  into the form for re-submission as POST (plus token, added by HTMLForm).

* Remove the distinction between WebUI and API tokens. This stronger token salt made it
  unnecessarily complex and was only there because it used GET until now. This streamlining of
  tokens matches what we already do for 'watch', 'edit', 'patrol' and other actions.

* Fix form submission bugs when 'from' query parameter is missing.

  - Ensure the required 'from' query parameter is present before showing a form.
    No need for the user to submit a form we know will fail.

  - Plain GET request to action=rollback (with no parameters) is now a 400 Bad Request
    instead of a form that would fail when submitted.

  - Submitting the form without 'form' field now correctly says why it failed.
    Previously it emitted a session error, which was a lie.

Bug: T88044
Change-Id: Ia457802fec2e90573c8e7d552bc1f3cee258f10b