refactor login as to be impersonation with better login/logout
Modifies the existing "login as" feature to be called impersonation.
This also adds:
* Application keep track of who is impersonating the user so they can revert back to the original user without having to log out.
* Stores the user profile via `HTTP_REFERER` so you get redirected back to the person you have impersonated once you stop.
## Screenshots:
See merge request !1702
