Add the jsonpatch package
Add a function to supplement versions with diffs between them

This uses jsonpatch to generate a patch we can render like a diff.
Render difference between versions, not just the whole version

This also hides the full version data unless you click 'Show'

This is a bit ugly, but we should have some designer time soon to tidy
it all up.

Fixes #39
Merge branch 'show-version-diffs'