Fix incorrect autocompletion and missing license check in related items UI

What does this MR do and why?

This MR fixes a couple of bugs as a follow-up for !81119 (merged):

• A bug where # would autocomplete issues within related epics UI, and & would autocomplete epics within related issues UI.
• Missing check for related_epics feature availability in EE license while rendering related epic UI.

How to set up and validate locally

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

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 #356013 (closed), #353170 (closed)