Update merge request widget to stop polling

What does this MR do and why?

Updates the merge request widget to stop the polling requests. Most of, if not all of, the state changes that happen in the widget are controlled through the data received from the Apollo subscription so we are currently just hitting the widget endpoints because of legacy reasons.

This change however is being done behind a disabled feature flag due to the impact this change could have if something is missed.

References

Screenshots or screen recordings

How to set up and validate locally

  • Enable the merge_widget_stop_polling feature flag
  • Visit a merge request
  • Check the network tab in dev tools to make sure no polling requests happen

MR acceptance checklist

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