Expand all button on merge requests diff view not working properly

Summary

This is a followup to https://gitlab.com/gitlab-org/gitlab-ce/issues/51737 which was fixed with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22873 .

Even though the button is now visible it still doesn't work like it use to.

Steps to reproduce

1. go to a mr diff page, eg https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21089/diffs
2. click on the Expand all button

What is the current bug behavior?

The button uncollapses only loaded files, while all files that have not been loaded still have This diff is collapsed. Click to expand it.

What is the expected correct behavior?

The button used to add a expanded=1 parameter to the url which would reload the page with all files loaded & uncollapsed. This behavior is still present on the commit view, eg https://gitlab.com/gitlab-org/gitlab-ce/commit/d931607f41f6626e48d2f4510ec4c6c848386d07 .