Watchlist: Fix form and preference overriding

Watchlist: Fix form and preference overriding

Fix a regression caused by aa063f4c5a19.  Restore the behavior
where:

* Without action=submit: Boolean preferences apply to the displayed form and its
  results, but they can be overriden by the query string.
* With action=submit: Only boolean parameters in the query string apply;
  preferences are not considered.

  (However, boolean preferences not on the form, i.e. 'extended', still
  always apply.)

This behavior is a consequence of how checkboxes work in HTML; only
checked boxes are sent to the server.

Bug: T160734
Change-Id: Ic050dd1445ade9449496bb051f04ca0a87b8b5d7