Add faces.py, taken from SayIt's thumbnail processing

This is (a very slightly changed version of)
speeches/thumbnail_processors.py from SayIt.

This works well for cropping images to just to the face of the person in
the photo.
Update the PPC scrapers to also fetch and auto-crop images
Switch 'image_for_cropping_url' for 'image_url' in the PPC JSON
Add scraped and cropped PPC images
Add some updated files from the recent scrape
Remove LibDem placeholder images
Fix miscropped images
Switch the Labour scraper to output image_url too
Update all Labour PPC with s/image_headshot_url/image_url/
Remove a redundant file from the old Labour scraper