Improve HTMLForm (and Special:ChangeCredentials) cancel button

Improve HTMLForm (and Special:ChangeCredentials) cancel button

Add two new HTMLForm methods:
* showCancel( [bool] ) to display a cancel button
* setCancelTarget( Title|string ) to set where it should take
  the user.

The cancel button is a simple link formatted as a button.
This is faster than a real button (skips an unnecessary
submit and redirect) and avoids some UX problems:
* when clicking on a real button, HTML5 or JS  validators
  might prevent submission, which does not make sense for cancel
* form field values might get saved by the brower, which again
  does not make sense for cancelling

Use the cancel button for Special:ChangeCredentials.

Bug: T136809
Change-Id: Ieb80e2ff36751abc6f00e2a02926fe9800666a8b