Update clearance to 1.2.x

Even after removing the references to assign `current_user` we still get WARNINGS:
'Assigning the current_user this way has been deprecated. You should instead use the sign_in method.'

clearance 1.2.0 includes https://github.com/thoughtbot/clearance/pull/376 which fixes the `current_user=` on `sign_in_as`.

This also changes the way we assign the flash message, to use the new way advised by default clearance controllers.

review @sferik 
cc @havenwood @derekprior