Make issue threads resolvable

What does this MR do and why?

Make issue threads resolvable just like MR threads

Related to #31114 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the resolvable_issue_threads feature flag
2. Create threads on an issue
3. Verify that "Resolve" / "Unresolve" buttons are present and work as expected

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.