Add email_participants widget to tasks

What does this MR do and why?

Add email_participants widget to tasks

If an issue has external email participants and it is converted to a task, the task still has the same external email participants. However, this information is not exposed so on the UI a task does not state anywhere it has external email participants. By adding the email_participants widget to tasks, tasks will show the external email participants on the UI correctly.

Changelog: added

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to issue
2. Add external participants with /add_email
3. Convert the issue to a task
4. Confirm the comment box still lists the external participants in the UI

MR acceptance checklist

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