Move polling of the MR Widget GraphQL endpoint into Apollo

What does this MR do and why?

For #384199 (closed)

Per the title: Moves polling for the widget state into the Apollo implementation.
The polling was already refetching the GraphQL data, this just manages it from within Apollo.

Screenshots or screen recordings

N/A, it's all backstage stuff.

How to set up and validate locally

1. Visit the Overview page of an MR
2. Note that the MR status (e.g. "Ready to Merge", etc.) loads immediately from a GraphQL batched query
3. Confirm that periodic GraphQL polling occurs in lengthening intervals: instant (this is the vue-apollo bug), 10s, 20s, 40s, 1m20s, etc. to a maximum value of 2m10s.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.