Repository Blob viewer: Fallback to highlighting Python syntax with Rouge

Python syntax is not being highlighted correctly on the Repository Blob Viewer in some cases.

The reason for this is the way we use Highlight.js (highlighting in chunks) can cause Highlight.js to lose context of what it is busy highlighting.

As an interim solution, we can fallback to highlighting Python code with Rouge (highlighting on the backend). This would alleviate the pressure and allow us to get to a better solution.