Blob refactor: Fix sourcegraph integration

What does this MR do and why?

This ensures that we load Sourcegraph when navigating from the repository or tree controller.

Changes in this MR is part of bigger efforts to refactor the blob viewers and Highlight source code syntax on the frontend using HighlightJS.

Screenshots or screen recordings

How to set up and validate locally

Note, to verify locally you'll need to setup Sourcegraph (step 3 below)

1. Enable the Blob Viewer Refactor feature flag

Feature.enable(:refactor_blob_viewer)

2. Enable the Highlight.js feature flag

Feature.enable(:highlight_js)

3. Follow the steps in docs for setting up Sourcegraph

4. Open a source file

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.