Split database migrations into pre- and post-attachment migration

- Run light-weight migrations before attachment migration.
- Run regular migrations after attachments have been moved to disk.

js/modules/migrations/migrations_1_database_without_attachment_data.js

@@ -0,0 +1,9 @@
+exports.migrations = [
+  {
+    version: 18,
+    async migrate(transaction, next) {
+      console.log('Migration 18');
+      next();
+    },
+  },
+];