Adds email participants widget definition

What does this MR do and why?

We already added the email participants widget in !162411 (merged) but the data migration has been reverted in !165219 (merged) due to migration errors with a renamed index.

This change adds the widget definitions for the email participants backend widget in a post deploy migration.

Contributes to Work item quick actions: add_email and remove_e... (#462118 - closed) so we can use the external participants feature from Service Desk on work items.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

backend only changes

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. bundle exec rails db:migrate to run migration
2. bundle exec rails db:rollback:main to revert migration (only mail affected)