Story 1280 (1/2): Overhaul of image uploading overlay design

* Separate styles for the new overlay.
* Use LoadingOverlayNew during dynamic loading of new uploading overlay.
* Change license copy from "By clicking Submit..." to "By continuing"
  since the new upload button has no caption.
* Change new overlay header CSS to display: table for better
  flexibility.

Change-Id: Ie00b8d76caae882ae27e726478d1662fcced63d6
Merge "Story 1280 (1/2): Overhaul of image uploading overlay design"