Remove isLightTheme GraphQL flag from codebase

Created by: eseliger

Also, adds a GraphQL deprecation annotation to the flag, so customers are more likely to discover it. After this PR, we won't need to reload blobs after a theme change. I originally started this effort in https://github.com/sourcegraph/sourcegraph/pull/20625, but the diff became stale so started over here. Lot's of the things from the other PR have already been done by now.