db: add a kind for extensions

migrations/kind_2_down.sql

@@ -1,3 +1,6 @@
-DELETE FROM kind WHERE id = 'r';
-DELETE FROM kind WHERE id = 'c';
-DELETE FROM kind WHERE id = 'a';
+DELETE FROM kind WHERE id IN (
+	'x',
+	'r',
+	'c',
+	'a'
+);

migrations/kind_2_up.sql

@@ -1,3 +1,4 @@
 INSERT INTO kind(id, name) VALUES ('a', 'author');
 INSERT INTO kind(id, name) VALUES ('c', 'character');
 INSERT INTO kind(id, name) VALUES ('r', 'release');
+INSERT INTO kind(id, name) VALUES ('x', 'extension');