Adding build_media management command
Adding build_media management command

I think it's cleaner if these things are management commands instead of
separate commands like .docker/bin/build_media.  It's easier to
add/remove a management command than mess with the entrypoint.

Keeping the docker script for now, since we're still using it with our
current deployment code.

pculture/unisubs,pculture/unisubs,pculture/unisubs,pculture/unisubs