Add xapian to the mailman-web container

Whoosh completely breaks down on 4GB of mail archives, so provide xapian
as an alternative. It's not hooked up automatically, but can be enabled
in the config files.

Related to issue #97.

web/Dockerfile

@@ -15,7 +15,7 @@ RUN set -ex \
 		postgresql-dev mariadb-dev python3-dev \
 	&& apk add --no-cache --virtual .mailman-rundeps bash sassc \
 	   postgresql-client mysql-client py-mysqldb curl mailcap \
-	   python3 py3-setuptools \
+	   python3 py3-setuptools xapian-core xapian-bindings-python3 \
 	&& python3 -m pip install -U django==2.1.5 pip \
 	&& python3 -m pip install mailmanclient==3.2.1 \
 		postorius==1.2.2 \
@@ -27,6 +27,7 @@ RUN set -ex \
 		dj-database-url \
 		mysqlclient \
 		typing \
+		xapian-haystack \
 	&& apk del .build-deps \
 	&& addgroup -S mailman \
 	&& adduser -S -G mailman mailman \