JSON normalization followup

[danvk]

Minor nice-to-haves after the core functionality in #237:

• [x] Add a keyboard shortcut (n and j are the obvious choices, but they're already taken; n=next diffhunk, j=nextfile)
• [ ] No loading indicator when toggling normalization on/off
• [ ] Only show the toggle if there's a diff
• [ ] Only show the toggle if it would make a difference
• [ ] Report diffstats after normalization
• [x] Show the "no diff" banner if there's no diff after normalization
• [ ] Add a way to default this to true (workaround: add ?normalize_json=1 to the URL or toggle under the gear)
• [x] Match the visual style of .image-diff-controls (gray background, full width)
• [x] Add a way to toggle this under the settings gear, so that you can set it before navigating to a big JSON file.
• [x] Suppress diffs for big, one-line JSON files. (Example: https://github.com/danvk/oldnyc/commit/172b3b276de902133a290dc350adb5ea79cb20b2)
• [ ] Offer normalization for pure adds (Example: https://github.com/danvk/oldnyc/commit/7c30363af5d7324603184e0ebe8ced93260ebdf2)