Don't include people who've changed constituency in the 'not sure' section

Previously, people would only have been excluded from the "not sure"
section on the constituency page if either they're marked as standing
again in that constituency, or they're marked as not standing anywhere -
it was missing people who we know are standing in a different
constituency.

Fixes #88
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'
Don't include people who've changed constituency in the 'not sure' section

Previously, people would only have been excluded from the "not sure"
section on the constituency page if either they're marked as standing
again in that constituency, or they're marked as not standing anywhere -
it was missing people who we know are standing in a different
constituency.

Fixes #694
Add scraped and auto-cropped PPC images
Add the unknown person icon from TheyWorkForYou
Add more manual matches of new candidates against old
Update the submodule with scraped PPC data
WiP: display candidate images (ropey CSS)

We should have some designer time this week to look at making this
actually look good, so this is just Work-in-Progress.
WiP: add more human decision about PPCs