Drop support for multiple subadressing

Change format from notifications+<channel-name-slug>+<level>@dovecotmta.cern.ch to notifications+<channel-name-slug>.<level>@dovecotmta.cern.ch

Create follow tickets where needed: backend, consumer, docs and api

Nothing to do in email_gateway. Fix has to be done in consumer email_gateway: edit EMAIL_RECIPIENT_REGEX