Fixed the edX middleware to lazily create request
context for Mako templates, so that it is based on
any changes made to the request object.
Verified that with this fix the "Instructor" tab is
correctly hidden when a staff member is viewing
the course as a student.
This code adds the ability to add Mako template lookup directories on
the fly, allowing third party add-ons to contribute their own Mako templates.
A new API function for registering Mako templates is introduced::
from edxmako import add_lookup
add_lookup('main', '/path/to/templates')
# Or, specify a package to lookup using pkg_resources. This will
# add the 'templates' directory inside the current package:
add_lookup('main', 'templates', package=__name__)
