Get rid of uses of the deprecated 'msg.connection'

Closes #404 and #413
On second thought, I think it's correct as is. `msg.connections[0]` is a Connection model instance which has `contact` as an attribute, which will always be present. It's allowable for `connection.contact` to be `None`, so we don't want to throw an exception in that case. Note that the previous code was only trying to ensure that at least one of contact or connection was provided, which we do by just checking for `connections`.