Performance: Preload file blobs on search

Created by: umpox

Actions before merging this PR:

• Use a new feature flag
• Merge https://github.com/sourcegraph/sourcegraph/pull/40344 so there's no significant change to fetching data (similar KB fetched by user).

Ideally also merge https://github.com/sourcegraph/sourcegraph/pull/40244 so this is really fast for new repos/revisions too. But this isn't required necessarily.

Description

This PR:

• Starts preloading plaintext file blobs on search submission, when the result becomes visible (through the VirtualList)

Before

https://user-images.githubusercontent.com/9516420/184664937-b8093cc9-6502-4ce4-bb2b-3442df28754a.mp4

After

https://user-images.githubusercontent.com/9516420/184665005-8431d02d-f35f-4b46-8e3d-df2cde6b63e3.mp4

Test plan

App preview:

• Web

Check out the client app preview documentation to learn more.