Fix Special:ListUsers "subpage parameter" handling

* Set the form action to avoid the subpage being "stuck". This is
  apparently a common problem and a common workaround, hmm. The $self
  variable, curiously unused, seems to have been meant for this.
* Fix incorrect parameter name to display values from subpage
  parameters in the form, and preserve them after submission.

Bug: T171657
Change-Id: Ibeee545a1e9932dfe44165c34cb55ad87268b705