Add support for full airgram services

Airgram support currently uses send-as-guest, but if you want to make a full fledged airgram service and use that (allowing for a custom title and icon and other niceties), this commit adds airgram-service as a separate service. It contains support for sending messages and sending the mandatory subscription request.

Note that it keeps the same nomenclature of putting the email address in the target field as the existing airgram service does, even though technically it should be in 'username'. 'username' is thus used to store the service key. 'secret' is for the service api secret.

This service has been extensively tested with a sample size of 1.