Fixed MR widget unresolved discussions state

What does this MR do?

Fixes the merge request widgets unresolved state not being updated after a discussion was resolved.

This can cause an issue where the user needs to refresh the page if they have the Only allow merge requests to be merged if all discussions are resolved option enabled. Before the Vue refactor this would get updated after resolving so this merge request restores that behaviour.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/42882

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides