Fix notes count on MR to not increment when notes are loading

What does this MR do and why?

Before this fix, when there are multiple pages of discussions, it's noticeable that the notes count on the MR page will increment while discussions are getting loaded.

In this fix, we only update the count when all the pages of discussions are done loading.

This is behind paginated_mr_discussions feature flag.

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Related to #368216 (closed)