Revert "Use {% static %} for all images"

I don't know what I was smoking, sorry. Media files should not be passed
through the {% static %} tag. And the url function of ImageFields
handles handing out the correct URL just fine.

This reverts commit 9f5301e6f50a8c129a29c69f8cc64034643228e3 and
c9576c9ca0cccb624339131c7cdb4a0b628cd576?.
Remove unnecessary missing image handling

That code did nothing.
Use thumbnails for ImageInput widget

The entire image handling in the dashboard isn't very pretty. But at
least we're not using full-size images for this.
We also can't assume that just prepending MEDIA_URL is the correct URL
for every image (e.g. not when using S3 backends). Passing it through
Sorl gets us the correct URL for free.
Merge pull request #1042 from tangentlabs/fix/media

Revert {% static %} tag for media files & tweaks