These changes make to possible to run:
tutor mounts add /path/to/my-xblock
The xblock directory with then be auto-magically bind-mounted in the
"openedx" image at build time, and the lms*/cms* containers at run time.
This makes it effectively possible to work as a developer on
edx-platform requirements.
We take the opportunity to move some openedx-specific code to a
dedicated module.
Close https://github.com/openedx/wg-developer-experience/issues/177
