Improve image uploader UI

closes #1356
- add button to toggle back to uploader from image-url interface
- put save button next to url-input
- make box-height consistent
- position ui-elements to have equal padding all around to border

![ghost_image_uploader_ui mov](https://f.cloud.github.com/assets/7757/1606849/891e9184-545e-11e3-846f-4861f207af3c.gif)