JSON normalization followup

[danvk]

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

• 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
• 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)
• Match the visual style of .image-diff-controls (gray background, full width)
• Add a way to toggle this under the settings gear, so that you can set it before navigating to a big JSON file.
• Suppress diffs for big, one-line JSON files. (Example: danvk/oldnyc@172b3b2)
• Offer normalization for pure adds (Example: danvk/oldnyc@7c30363)