mailman3: Switch to a faster search backend (Xapian)

Whoosh is used by default, but it is slow at indexing (multiple hours
for just aur-requests) and searching e.g. aur-requests isn't possible
(it is slow and uses 3G+ of memory resulting in it getting OOM-killed).

Xapian indexed everything in just 76 minutes and searching aur-requests
now works and is plenty fast.

Co-authored-by: Evangelos Foutras <evangelos@foutrelis.com>