Restore full-text search with Solr

The configuration file `90-plugin.conf` exists but was not getting
installed.

roles/mailserver/tasks/dovecot.yml

@@ -46,8 +46,9 @@
     - 10-auth.conf
     - 10-mail.conf
     - 10-master.conf
-    - 90-sieve.conf
     - 90-antispam.conf
+    - 90-plugin.conf
+    - 90-sieve.conf
     - auth-sql.conf.ext
   notify: restart dovecot