Add Roles as sub-tab within the User tab

[mtomov]

• probably not the ideal solution to specifiy a partial, as if other

  extensions specify one, or solidus itself specifies one, we'd be

  overriding it. Oh well, that's for a different day.

[acreilly]

I had done this: https://github.com/boomerdigital/solidus_user_roles/commit/d51182edf717120ea2776db647e1b9350779af46

Not as pretty but does the job.

[mtomov]

No, actually your way is better, as if someone else decides to add a partial underneath User, this one will break. I'd prefer yours!