Take initial steps to update the design of the search panel and filter controls
to align with the mocks in https://github.com/hypothesis/client/issues/6006.

 - Move the filter status UI into the search panel. This reduces the amount of
   vertical space required when a search and/or filter are active.

 - Replace the filter status description string + single filter state toggle
   with a row of toggle buttons, one per filter.

   Using one toggle button per filter allows the user to toggle the different
   filter states independently. Removing the filter description saves space and
   also it was getting increasingly complicated to construct descriptions of all
   the possible search/filter combinations.