Experiment with caching Rapid Diffs HTML by the MR version

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We have an excellent caching POC by @patrickbajao which uses diff file paths to cache Rapid Diffs HTML file by file.

This approach has a downside that it needs to get the contents of the diff files before it can return the cache. We should experiment with another approach: create a cache key using MR ID and MR version to cache the HTML of the first 5 files and the rest of the files. We expect this to produce much better performance improvements: current POC only produces 19-22% improvement in response time.