S3 based file uploading

- [ ] Ready for merge

Allow pro users to drag and drop assets inline to JS Bin and uploads to S3.

- [x] Direct upload to s3
- [x] CodeMirror insert feedback (ala github)
- [x] Textarea insert feedback (ala github)
- [x] JS, HTML and CSS support
- [x] Preprocessors (likely to be dropped)
- [x] Account panel to "manage" assets (at least view)
- [x] Ability to remove assets
- [x] Double check security