Updated merge request dashboard to hide drafts

What does this MR do and why?

Adds a draft toggle to the config dropdown that will allow users to hide/show drafts on the your merge request list on the merge request dashboard.

References

#551475 (closed)

Screenshots or screen recordings

How to set up and validate locally

• Enable mr_dashboard_drafts_toggle feature flag
• Visit https://gdk.test:3000/dashboard/merge_requests
• Config dropdown should have toggle for hiding drafts
• Your merge requests list will hide the drafts when this is toggled off

MR acceptance checklist

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