Add repository storage field back to projects API

Merged Sean McGivern requested to merge add-repository-storage-back-to-api into master

This was an admin-only field that was exposed, and there were no tests for doing this as an admin and getting that response. In CE, we renamed the user option to current_user, and inadvertently broke this.

Reference: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7754#note_19251903