Scalastyle linting in admin submodule.

Public method must have explicit type.
Implement UserMixin.

User collection and its functionality can be used in several
controllers, and for reusability they should be separated into an
independent mixin.
Change User controller's failure results to `BadRequest`.

Returning `Ok` for failure cases doesn't make sense. They're changed to
`BadRequest` with a useful JSON body.
Implement user login handler.

Until now a user's password hasn't been checked for login. This patch
implements the login functionality including checking a username and
password.
Implementing testcase...