WIP: xblock pipeline work (#10176)

[PERF-303] Integer XBlocks/XModules into the static asset pipeline.

This PR, based on hackathon work from Christina/Andy, implements a way to discover all installed XBlocks and XModules and to enumerate their public assets, then pulling them in during the collectstatic phase and hashing them.  In turn, the methods for generating URLs to resources will then returned the hashed name for assets, allowing them to be served from nginx/CDNs, and cached heavily.

common/lib/xmodule/xmodule/error_module.py

@@ -75,6 +75,7 @@ class ErrorDescriptor(ErrorFields, XModuleDescriptor):
     Module that provides a raw editing view of broken xml.
     """
     module_class = ErrorModule
+    resources_dir = None
 
     def get_html(self):
         return u''