Updated merge requests nav count for drafts setting

What does this MR do and why?

Updated the merge requests navigation count to take into account the users show drafts toggle setting.

References

#551475 (closed)

Screenshots or screen recordings

How to set up and validate locally

• Enable mr_dashboard_drafts_toggle feature flag
• Have at least 1 draft merge request assigned to your user that does not have any reviewers assigned
• Visit https://gdk.test:3000/dashboard/merge_requests/ and toggle the Show your drafts preference
• The merge request count in the navigation bar should be excluding the number of drafts assigned to your user

MR acceptance checklist

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