Create a thumbnail for each uploaded video

Also fixes opengraph support that was lost in the redesign.

We may want to change how this works. The converter for png images only
works with video files as the input. Might be worth tweaking it so that
we explicitly declare which input file type this converter is meant to
handle.

For what it's worth, we can convert other images into PNG files with
imagemagick like this: `convert input.ext output.png`
Merge branch 'master' into thumbnails

Conflicts:
	templates/layout.html
Merge branch 'master' into thumbnails

Conflicts:
	mediacrush/worker.py
Merge branch 'master' into thumbnails

Conflicts:
	templates/view.html
Declare certain processing steps as "extra"

These don't get factored into the compression calculation, and they
don't represent the same media. This is currently used to make video
thumbnails distinct from the videos themselves.
List 'extra' files in /hash.json
Merge branch 'master' into thumbnails
Mention 'extra' files in API docs
Removed 'extras' as a requirement in the processing dict
Merge pull request #344 from MediaCrush/thumbnails

Create a thumbnail for each uploaded video