Code Insights: add polling when fetching an InsightView and the insight is backfilling

Created by: chwarwick

When attempting to fetch an insightView and that view is currently backfilling data, the web app should start polling until the backfill process has completed so that the user is presented with the complete view as soon as possible.

Future changes in the API will mean that all search insights will get backfilled data points instead of running the queries just in time. This change is meant to allow those insights that run only over some repos to change from the This insight is still being processed state to a full display quickly and without any additional user interact.

Acceptance Criteria:

• When backfilling is complete the This insight is still being processed is removed without user intervention and the completed chart is visible
• Webapp polling of backfilling insights can be disabled.