CE backport of changes to suppport group file templates

What does this MR do?

Backports changes in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7391 to CE.

There are no visible behaviour changes in this MR. We add some extra JS to the group settings pages, but it doesn't do anything in CE.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/5987

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides