Drop support for multiple subadressing

from email-gateway#19 (closed)

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
• api

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

Logs show only 1 feed notifications+cds-team+important@dovecotmta.cern.ch in the last 15 days