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