Add Related Epics support within epics

What does this MR do and why?

Adds support for Related epics within an epic, this feature behaves identical to Linked issues feature.

Screenshots or screen recordings

Default view	Add epic	Epics autocomplete	List of linked epics

How to set up and validate locally

1. Enable :related_epics_widget feature flag for any group from Rails console (opened by running bin/rails console.

   Feature.enable(:related_epics_widget, Group.find_by_full_path('gitlab-org'))

2. Checkout this MR's branch 353170-add-related-epics-frontend locally.
3. Visit any Epic within gitlab-org group (where we enabled the feature-flag)
4. You should now be able to add epics as related as shown in the screenshots.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #353170 (closed)