Support for storing an XBlock's static asset files in Blockstore

This PR introduces some backend python + REST APIs for storing static
asset files along with an XBlock in a content library. It also updates
the new runtime to be able to load such static asset files.

Example use cases:
* Store an image file with an HTML block and then use the image inline
  in the HTML block.
* Store a PDF file with an HTML block and provide a link in the HTML for
  the learner to download the PDF.
* Store .srt files or even video .mp4 files that belong to a video
  XBlock.

Within the bundle, these static asset files are stored in a "static/"
subfolder of the folder that contains the OLX file. Extending an
existing LMS/Studio convention, a static asset file such as "image.png"
is referenced within the OLX as "/static/image.png" and the URL will be
rewritten by the runtime.