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
Modifies the existing "login as" feature to be called impersonation, as
well as keeping track of who is impersonating to revert back to that
user without having to log out.
Animation of Sidebar (video in here)
Edited the CSS of header and sidebar; added a new div of gitlab-logo-container to the html to make things easier.
The transition-duration is set to 0.3s and when the nav bar expands, it opens up over the text, essentially revealing hidden text.
[animation-sidebar-2.mov](https://gitlab.com/gitlab-org/gitlab-ce/uploads/b15996a256d6bed03a19c68549d116e4/animation-sidebar-2.mov)
/cc @JobV @nicolas2
See merge request !849
