Include external participants from the `To` header
From #4652 (comment 1916792621)
We are able to repro the case where an external participant emails the Gitlab bot address, but instead of using CC: just adds another email in the To: line. Gitlab does just ignore this. I don't think it's a safe assumption that people are always going to use CC since there is no functional difference to most people between adding an email on 'to:' versus 'cc:' so that does sound like a bug.
Proposal
If configured, also include email addresses from the To
header as external participants.
Additional points:
- Maybe revise whether we need to rename the table column
- Change text in UI
- Change documentation