Indicate specific channel to use in Nuntium calls

[matiasgarciaisaia]

When asking Nuntium to send a message, the channel matching is currently being done by application ID.

Since it is Pollit that creates the channel in Nuntium, it should use the somewhat new feature that enables to specify the channel to use rather than hint it.

Failing to do so can lead to the wrong channel being picked up if someone creates a non-restricted channel in Instedd's Nuntium account - which Pollit end-users can't see or troubleshoot at all.