www_listing: support /all/ search as a 302 redirect
This allows users to search /all/ from the top-level WwwListing
without extra manual steps, although there's still extra network
roundtrips incurred.
No vertical whitespace is added, and there's no clumsy radio
buttons nor menus to deal with. Users only have to use a
different <input type=submit /> button. I forgot how to do this
until I realized we already do something similar with multiple
submit buttons for threaded vs non-threaded mboxrd.gz downloads.
Link: https://public-inbox.org/meta/20210827120845.29682-1-e@80x24.org/