Fix bango logout (bug 1105076,1105076,1049004)

* Use logged_in_user as the key for provider logouts. 
* Set the localStorage to a logout+timestamp when we logout (This causes the provider logout later at the point we know the provider).

r? @kumar303 @washort 
`null` is returned for a non-existent item so in this case that will be false which parses fine.