Fix discussions filter not working

What does this MR do and why?

This bug was introduced by !69933 (merged) when we changed the mutation to append or update discussions.

When changing the discussions filter, we must clear the existing discussions before fetching the filtered list.

This also disables the filter when discussions are loading to prevent race conditions where the response of a previous request could show up if the filters are switched very quickly.

How to set up and validate locally

1. Open an issue / MR and change the discussions filter to "Show comments only" / "Show history only"

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.