FIX: Allow themes to upload and serve js files (#8188)

If you set `config.public_file_server.enabled = false` when you try to get uploaded js file you will get an error:
`Security warning: an embedded <script> tag on another site requested protected JavaScript. If you know what you're doing, go ahead and disable forgery protection on this action to permit cross-origin JavaScript embedding.`

The reason is that content type is `application/javascript` and in Rails 5 guard looked like that:
https://github.com/rails/rails/blob/5-2-stable/actionpack/lib/action_controller/metal/request_forgery_protection.rb#L278-L280
However, in Rails 6 `application` was added to regex:
https://github.com/rails/rails/blob/master/actionpack/lib/action_controller/metal/request_forgery_protection.rb#L282-L284

This pull request is related to https://meta.discourse.org/t/uploaded-js-file-for-theme-causes-a-rejection/129753/8

spec/fixtures/themes/test.js

@@ -0,0 +1 @@
+console.log("test");