Add Flask-Migrate to the project

Also moved old migrations to old_migrations folder (before deleting them entirely).

old_migrations/0027_add_draft_toggle.py

@@ -0,0 +1,18 @@
+from playhouse.migrate import PostgresqlMigrator, migrate
+
+from redash.models import db
+from redash import models
+
+if __name__ == '__main__':
+    db.connect_db()
+    migrator = PostgresqlMigrator(db.database)
+
+    with db.database.transaction():
+        migrate(
+            migrator.add_column('queries', 'is_draft', models.Query.is_draft)
+        )
+        migrate(
+            migrator.add_column('dashboards', 'is_draft', models.Query.is_draft)
+        )
+        db.database.execute_sql("UPDATE queries SET is_draft = (name = 'New Query')")
+    db.close_db(None)