"users" is a valid username and conflicts with an unused route

/users/... is clearly a reserved top-level route:

• https://gitlab.com/users/sign_in
• https://gitlab.com/users/auth/google_oauth2

But /users by itself is unused, and surprisingly, "users" is a valid username.

Arif has apparently figured this out: https://gitlab.com/users

It seems like "users" should be a forbidden username, as /users by itself may become useful.