Add a single email participant via quick action
Only possible with issues with no email participants yet.
- Only project members (with guest+ access?)
- Preview the action (see designs in #195525 (designs)) with errors if the action can't be done (ie. another email is already participating, email not valid)
service_desk_reply_toas the provided email
- Add a system note
- Notify the participant that they have been added to the issue
Use feature flag