[UX] Search existing hits while waiting for results

[justinkambic]

Is your feature request related to a problem? Please describe.
Today, when I type a query into the search bar in the User Experience dashboard, the "top pages" items disappear and I see a loading dialogue. It's possible one or more of the top pages I wanted to see was already in the result set, or was further down the fold.

Describe the solution you'd like
We should continue to display the top pages results that match the user's query (if any), while we wait for the server results. We could keep these items displayed alongside the loading indicator, and when we receive results from Elasticsearch we can likely discard them, as they should be in the result set.

Describe alternatives you've considered
I had thought about merging the two lists of hits (after filtering the stale items based on user input), but these items should also exist in the result set, so this would be redundant.

Additional context
N/A