init commit

store/migration/sqlite/0.13/00__memo_relation.sql

@@ -0,0 +1,7 @@
+-- memo_relation
+CREATE TABLE memo_relation (
+  memo_id INTEGER NOT NULL,
+  related_memo_id INTEGER NOT NULL,
+  type TEXT NOT NULL,
+  UNIQUE(memo_id, related_memo_id, type)
+);