Adds frontend support for realtime updates on the merge request homepage

What does this MR do and why?

Adds 1 subscription handler that will wait for any change to any merge request the current user is an assignee or reviewer of. If there is an update to these we trigger a refetch of the lists that it affects. If any merge request gets added to the list we add a green background. The green background will remain until the next realtime update. There is also a visibility check, if the user isn't viewing the page and multiple realtime updates come in we won't clear the green background for older updates, we only clear it again the user has viewed the page and another realtime update comes in.

References

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• #512629 (closed)

MR acceptance checklist

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Screenshots or screen recordings

How to set up and validate locally

• Enable merge_request_dashboard feature flag
• Enable merge_request_dashboard_realtime feature flag
• Visit http://gdk.test:3000/dashboard/merge_requests
• Go to a merge request in a different tab that you are assigned to and request a review
• The merge request should move list