Push highlight_js feature flag to frontend

What does this MR do and why?

Pushes the highlight_js feature flag to the frontend to give us the ability to check the FF when navigating from the tree view. This is needed for us to check the FF at blob_viewers/index.js when navigating from the tree view.

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

N/A

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