introduce environment variable to control dovecot full-text-search

core/dovecot/conf/dovecot.conf

@@ -21,7 +21,10 @@ mail_access_groups = mail
 maildir_stat_dirs = yes
 mailbox_list_index = yes
 mail_vsize_bg_after_count = 100
-mail_plugins = $mail_plugins quota quota_clone zlib fts fts_xapian
+mail_plugins = $mail_plugins quota quota_clone zlib{{ ' ' }}
+  {%- if (FULL_TEXT_SEARCH or '').lower() not in ['off', 'false', '0'] -%}
+  fts fts_xapian
+  {%- endif %}
 default_vsz_limit = 2GB
 
 namespace inbox {
@@ -39,11 +42,13 @@ plugin {
   quota_vsizes = yes
   quota_clone_dict = proxy:/tmp/podop.socket:quota
 
+  {% if (FULL_TEXT_SEARCH or '').lower() not in ['off', 'false', '0'] %}
   fts = xapian
   fts_xapian = partial=2 full=30
   fts_autoindex = yes
   fts_enforced = yes
   fts_autoindex_exclude = \Trash
+  {% endif %}
 
   {% if COMPRESSION in [ 'gz', 'bz2' ] %}
   zlib_save = {{ COMPRESSION }}