Revamped UX

A green progress bar as something uploads, a friendlier log, and an S3 credentials tester:

![upload anything into s3](https://cloud.githubusercontent.com/assets/4592/5005724/8b10af76-6a02-11e4-80a0-1dbb93f9ad97.png)

The state-in-permalink feature is removed (it didn't work), and there's some underlying refactoring in advance of a conversion to use localStorage for that stuff instead, but it's not there. I don't want this branch to become unstable or unwieldy, so I'm PRing it now.