Alerting: Adds support for sending a single email to all recipients in notification channel (#21091)
When an alert is sent by e-mail, the process sends an e-mail to
each recipient separately. This PR is a single delivery to all recipients.
For companies that use e-mail extensively, this is necessary in order
not to overload the sending queue.
Replaces #18013
Fixes #12650
Co-authored-by: Henrique Oliveira <holiiveira@users.noreply.github.com>
