Enables external participants for Service Desk
What does this MR do and why?
Contributes to [Feature flag] Rollout of `issue_email_particip... (#350460)
Feature epic: Participants in Service Desk (&3758)
Enables external participants for Service Desk (by default, release in 17.0
)
Users can add multiple external participants to a Service Desk ticket or regular issue and they are part of the conversation like the external author.
External participants receive Service Desk emails for every public comment on a ticket or issue and can reply to these notification emails.
Changelog: added
MR acceptance checklist
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