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

[Canapin (Coin-coin le Canapin)]

@jimkleiber

rather more so to impersonate a role/group member/trust level.

For the record, a trust level impersonation has been suggested here:

Give admins "Level_x" goggles

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.