Service Desk should add emails in the To line and CC line as external participants

Problem to solve

When emailing a service desk email, the author may include other email addresses to keep others informed. Other than receiving the initial email, these other participants do not receive any follow-ups from GitLab.

Proposal

• Any email address in the to and CC lines should be added as external participants.
• The author of the original email is the requester.
• Threading:
  • Each new email reply represents a new comment.
  • When replying in thread from GitLab, the author of the comment will be in the to line, other external participants else will be cc'd
• When creating a new comment in GitLab, the requester is in the to line, other external participants else will be cc'd