Support loading expanded context for discussions

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We should support loading expanded diff context in Rapid Diffs when a discussion was created on unchanged lines that were previously expanded.

The process could be the following:

1. Fetch discussions
2. Initialize the rendered diff
3. For each discussion: find all the code line ranges with discussions
4. For ranges that are outside of the current diff load the expanded lines (might need multiple passes)
5. Render discussions once the context is loaded