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

[Sander78 (Sander Datema)]

Well, wouldn’t mind that Discourse would track that.

Don't change 'last seen' when impersonating a user

When - as an admin - you impersonate a user, their last seen is updated. That is of course because you completely impersonate the user as if he were behind the keyboard. But… you’re not the user. You might use that option to quickly check something from the point of view of the user. So, is it possible to skip updating the last seen? And maybe there are other things that should not be updated when logged in through the admin account.