Add /users/:username syntax to the API

What does this MR do?

Now GET /users/4398 (by id) and /users/karlhic (remember that name) both work in the API.

Are there points in the code the reviewer needs to double check?

I wrote a test but couldn't run it because it crashed my chromebook.

Why was this MR needed?

Discussion: https://gitlab.com/gitlab-org/gitlab-ce/issues/13843

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #13843 (moved)