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.
• 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