Check related issues for format

Summary

When adding related issues, invalid user input is accepted by the frontend and then rejected by the backend.

Steps to reproduce

• open any issue
• enter things that are clearly not issues into related issues

What is the current bug behavior?

• input is accepted and sent to backend
• Add button looks disabled but remains clickable (even after removing invalid input)

not-related-issues

What is the expected correct behavior?

Invalid input is rejected by frontend.