Make location into a named tuple, and use it more as a first class entity, rather than URL for identifying content

Make user profiles work again after the switch to an XMLModuleStore. Staff user histograms are still broken

Begin using a Keystore for XML parsing. Still broken: sequence icons, custom tags, problems, video js

Move the resources_fs abstraction into the primary DescriptorSystem, as it is needed for more than just XMLParsing

Revert "Make import work via mako again, to unblock others while I work on making the LMS work using XModuleDescriptors"

This reverts commit 6fdf44fe.