New related work item via modal for Epics

What does this MR do and why?

Adds a new option to the action menu in the top right of the Work Item detail view which allows creating a new work item which is related to the work item you're viewing.

You can de-select the 'Relates to' checkbox to make a new work item without the relationship in place.

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.

Screen_Recording_2024-10-04_at_12.07.33

How to set up and validate locally

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

New related item

1. Visit an Epic
2. Click the 'New related item' option
3. Fill in the form and click 'create '
4. Check the new work item is related to the epic you were viewing.

New unrelated item

1. Visit an Epic
2. Click the 'New related item' option
3. Fill in the form and uncheck the 'relates to x' checkbox.
4. Click 'create '
5. Check the new work item is not related to the epic you were viewing.

Unauthorised user

1. Sign out
2. Visit an Epic
3. Check you cannot see the 'New related work item' option in the actions menu.

Relates to #478853 (closed)