Add a missing template.

templates/seed-empty-list-header-matches.py.j2

@@ -0,0 +1,28 @@
+"""Seed configured header matches on otherwise unconfigured mailing lists."""
+
+from json import loads
+
+from mailman.interfaces.mailinglist import IHeaderMatchList
+
+
+RULES = loads({{ mailman_empty_list_header_matches | to_json | to_json }})
+
+
+def apply(mlist):
+    """Add all configured rules if *mlist* has no header matches."""
+    header_matches = IHeaderMatchList(mlist)
+    existing_count = len(header_matches)
+    if existing_count:
+        print('SKIPPED {} existing_header_matches={}'.format(
+            mlist.fqdn_listname, existing_count))
+        return
+
+    for rule in RULES:
+        header_matches.append(
+            rule['header'],
+            rule['pattern'],
+            chain=rule.get('action'),
+            tag=rule.get('tag'))
+
+    print('CHANGED {} added_header_matches={}'.format(
+        mlist.fqdn_listname, len(RULES)))