Multi Image Upload

The upload UI has been given some styling and x-browser testing. Design-wise it's ready for review. Back-end wise @sixpearls code is still to be reviewed by a django dev.

IE9 doesn't support the `X-Requested-With` header, meaning as the backend code stands, no files are ever uploaded because the  `if request.POST and request.is_ajax():` clause is false. More importantly however it has no support for the File API, so progress indicators etc aren't supportable, which was the main benefit of this new upload system in the first place. tl;dr: it doesn't work at all in IE9.

My feeling is that getting it to all fall back nicely in IE9 is just too much work for too little benefit. I've become rather lost trying to figure out what would need to be done. I'd suggest we don't allow IE9 users to use this new upload system at all, redirecting them to the original instead. Users of IE9 will have `.lt-ie10` on the HTML tag which is easily detected and used as a basis for redirection either by meta tags or js.

Todo:
* [x] Replace existing image upload feature with this one
* [x] Consider changing "save" button (in post-upload form) to "update" instead. Otherwise it isn't clear your images have actually uploaded.
* [x] Provide fallback to old uploader for IE9
* [x] Add better feedback on invalid file uploads - should be done to whole image system, rather than Multi-upload specific.


The following will be tackled as part of a "phase 2" for this feature. They do not block merging:
* [ ] Add a field to `AbstractImage` to denote "incomplete meta data"
* [ ] Add some kind of help to upload screen, explaining the "incomplete" system
* [ ] Add dashboard listing of images awaiting metadata completion
* [ ] Update post-upload form to include "incomplete meta data" flag hidden field, set to "complete".
Can we bundle all of these image upload plugins together into a `{% compress js %}` tag?