Changes to handler URL generation

* The LMS now also monkey-patches
  xmodule.x_module.descriptor_global_handler_url and
  xmodule.x_module.descriptor_global_local_resource_url so that we can
  get LMS XBlock URLs from the DescriptorSystem. That functionality is
  needed in the block transforms collect() phase for certain XModules
  like Video. For instance, say we want to generate the transcripts
  URLs. The collect phase is run asynchronously, without a user context.

* The URL handler monkey-patching is now done in the startup.py files
  for LMS and Studio. Studio used to do this in the import of
  cms/djangoapps/contentstore/views/item.py. This was mostly just
  because it seemed like a sane and consistent place to put it.

* LmsHandlerUrls was removed, its handler_url and local_resource_url
  methods were moved to be top level functions. The only reason that
  class existed seems to be to give a place to store course_id state,
  and that can now be derived from the block location.

* To avoid the Module -> Descriptor ProxyAttribute magic that we do
  (which explodes with an UndefinedContext error because there is no
  user involved), when examining the block's handler method in
  handler_url, I made a few changes:

** Check the .__class__ to see if the handler was defined, instead of the
   block itself.

** The above required me to relax the check for _is_xblock_handler on the
   function, since that will no longer be defined.

90% of this goes away when we kill XModules and do the refactoring we've
wanted to do for a while.

common/lib/xmodule/xmodule/x_module.py

@@ -36,7 +36,6 @@ from opaque_keys.edx.asides import AsideUsageKeyV1, AsideDefinitionKeyV1
 from xmodule.exceptions import UndefinedContext
 import dogstats_wrapper as dog_stats_api
 
-
 log = logging.getLogger(__name__)
 
 XMODULE_METRIC_NAME = 'edxapp.xmodule'
@@ -1207,7 +1206,10 @@ class ConfigurableFragmentWrapper(object):  # pylint: disable=abstract-method
 
 # This function exists to give applications (LMS/CMS) a place to monkey-patch until
 # we can refactor modulestore to split out the FieldData half of its interface from
-# the Runtime part of its interface. This function matches the Runtime.handler_url interface
+# the Runtime part of its interface. This function mostly matches the
+# Runtime.handler_url interface.
+#
+# The monkey-patching happens in (lms|cms)/startup.py
 def descriptor_global_handler_url(block, handler_name, suffix='', query='', thirdparty=False):  # pylint: disable=invalid-name, unused-argument
     """
     See :meth:`xblock.runtime.Runtime.handler_url`.
@@ -1218,6 +1220,8 @@ def descriptor_global_handler_url(block, handler_name, suffix='', query='', thir
 # This function exists to give applications (LMS/CMS) a place to monkey-patch until
 # we can refactor modulestore to split out the FieldData half of its interface from
 # the Runtime part of its interface. This function matches the Runtime.local_resource_url interface
+#
+# The monkey-patching happens in (lms|cms)/startup.py
 def descriptor_global_local_resource_url(block, uri):  # pylint: disable=invalid-name, unused-argument
     """
     See :meth:`xblock.runtime.Runtime.local_resource_url`.
@@ -1821,6 +1825,7 @@ class CombinedSystem(object):
         DescriptorSystem, instead. This allows XModuleDescriptors that are bound as XModules
         to still function as XModuleDescriptors.
         """
+        # First we try a lookup in the module system...
         try:
             return getattr(self._module_system, name)
         except AttributeError: