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>

roles/mailman3/tasks/main.yml

 - name: Install mailman3 and related packages
-  pacman: name=mailman3,mailman3-hyperkitty,python-psycopg2,mailman-web,uwsgi-plugin-python state=present
+  pacman: name=mailman3,mailman3-hyperkitty,python-psycopg2,mailman-web,python-xapian-haystack,uwsgi-plugin-python state=present
   register: install
 
 - name: Install {mailman,mailman-web} configuration

roles/mailman3/templates/settings.py.j2

@@ -51,7 +51,7 @@ HYPERKITTY_ENABLE_GRAVATAR = False
 
 HAYSTACK_CONNECTIONS = {
     'default': {
-        'ENGINE': 'haystack.backends.whoosh_backend.WhooshEngine',
-        'PATH': '/var/lib/mailman-web/fulltext_index'
+        'ENGINE': 'xapian_backend.XapianEngine',
+        'PATH': '/var/lib/mailman-web/xapian_index'
     }
 }