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

I was in a state of mental confusion when I committed https://github.com/tangentlabs/django-oscar/commit/9f5301e6f50a8c129a29c69f8cc64034643228e3 and https://github.com/tangentlabs/django-oscar/commit/c9576c9ca0cccb624339131c7cdb4a0b628cd576. I think it resulted from a misunderstanding from what was causing https://github.com/tangentlabs/django-oscar/pull/825.

I've reviewed all use of {% static %} tags and slightly improved the ImageInput widget, which should address the issue in #825. 

I think it's good to merge, but I don't trust myself.