Various UI improvements

* Extensions are now sorted by publish date, newest first.
* Added a favicon.
* Upload display now shows messages on success/failure (with no hover). Layout of the upload boxes is optimized for our use case (no thumbnail).
* Fixed a bug in dropzone where clicking on the label wouldn't bring up the browse dialog.
* Changed the label to make it more obvious you can click.

The label should ideally be vertically centered in the box, but because this is CSS I couldn't figure out an easy way to do it :) (Specifically, setting `position: absolute; top: 50%;` doesn't seem to work because there's a min-height and no explicit height on the parent.)

@dangoor