♻️ Add minor refactor to file migrations

Relevant changes:

- Add the ability to create migration in both directions, defaulting
  to identity if not provided
- Move the version attribute to file table column for to make it more
  accessible (previously it was on data blob)
- Reduce db update operations on file-update rpc method

backend/test/backend_tests/rpc_file_thumbnails_test.clj

@@ -82,7 +82,6 @@
       (t/is (nil? (:error out)))
       (t/is (map? (:result out))))
 
-
     ;; run the task again
     (let [res (th/run-task! "storage-gc-touched" {:min-age 0})]
       (t/is (= 2 (:freeze res))))