Use Conference::can_pc_see_all_submissions() rather than pc_seeall.
'has:ext' search is equivalent to 'ext:>0'.

And similar for other review types.
Rearrange review settings to collocate deadlines.
Setting nits
has:cmt searches DO NOT search responses.

Same for has:aucmt/aucmt:>0 searches. This is a behavior change, but I
think a good one. Add anycmt: searches to search comments plus responses.
And document has: searches, which are nice I think.