Adds a new Studio-only libraries static asset endpoint at
/library_assets/blocks/{usage_key}. This endpoint will serve assets
only from the Draft branch, and is only available to people who have
read permission to the containing library.
This also moves the existing library asset endpoint that did lookups
by Component Version to /library_assets/component_versions/{uuid}
This change was motivated by the desire to make it easier to make the
editor preview work for images by having a single URL that will
consistently point to the latest version of the asset, rather than
having a new URL after every save (which the Component Version lookup
e67acd459f