Webapp should pre-fetch some search result highlights

Created by: slimsag

Currently the request flow for a search looks like:

• GraphQL search request -> results
• result.file[0] -> highlight file request
• result.file[1] -> highlight file request
• result.file[2] -> highlight file request
• ...

We could send back highlighted files as part of the initial GraphQL search request (for e.g. the first 10 results only) to reduce round-tripping latency from being induced. This would help improve search result load times substantially when there is a fair amount of latency between the client and server.

(this suggestion came from chatting with Keegan)

Related to (but distinct from): https://github.com/sourcegraph/sourcegraph/issues/6992