server/migrations: fix imports for alembic

`alembic revision -m 'blah blah'` rightfully complained about imports
(in case of `upgrade`, that module was being populated by some other
module.)

server/szurubooru/migrations/versions/9837fc981ec7_add_order_to_tag_names.py

@@ -7,6 +7,7 @@ Created at: 2016-08-28 19:03:59.831527
 
 import sqlalchemy as sa
 from alembic import op
+import sqlalchemy.ext.declarative
 
 
 revision = '9837fc981ec7'