Search results h2s and text filters

[janeforshort]

When someone does a search in the admin, the h2 still says Edit Posts/Comments or whatever the screen name is, and there's no visual indicator that these are search results. There should be a different header for this, something like Posts containing "[search term]".

Also, the text filters (All/Pending/Spam etc) still show, but they are not filtering the search results, they are links to the regular screens. In search results, filters should now show unless filtering the search results (which we don't do, so ought to just remove them).

[azaozz]

(In [10034]) Add search string to H2s, fixes #8373