Add 'stop impersonating' link to user avatar dropdown when impersonating

[Canapin (Coin-coin le Canapin)]

@jimkleiber

역할/그룹 멤버/신뢰 수준을 사칭하는 것이 더 많을 것입니다.

기록을 위해 말씀드리자면, 신뢰 수준 사칭은 여기에서 제안되었습니다:

관리자에게 "Level_x" 고글 제공

There have been a few times recently when I needed to see what something looked like from the perspective of a non-privileged user. What I’ve done was find an example user & “impersonate” them. When I’m done, I have to log out & back in. It would be nice if I could stay “me” and just temporarily downgrade what I’m seeing.