Persist diff notes highlighted diff

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Today we fetch blobs in a N+1 manner when viewing the Discussion tab in order to highlight the diff of each note. The same batch method (list_blobs_by_sha_path) currently works for /diffs tab, mainly because we're dealing with the same revision for every file, which is not the case when presenting discussion diffs

Having the notes highlighted diffs persisted (on NoteDiffFile) will avoid that.