Ask for confirmation when switching into an account which somebody else is already switched into

[johnbillion]

Fixes #94

Concerns

• If a user switches into an account and then goes on holiday for a week, other users who attempt to switch into the account will see this message every time they attempt to switch. Annoying, although not the end of the world. Perhaps check the session start time and only show the warning if the switch happened within something like the last 12 hours.
• This doesn't fully support stacked switching. I think that's too much of an edge case for this first version. (I'm questioning how useful stacked switching is, perhaps it needs to go).

Screenshots