Pluggable InputTypes use edxmako.paths.add_lookup to inject new
mako template directories. Those were getting clobbered by microsites
(and by theming), which were completely rebuilding the mako template
lookup list from scratch.

This commit switches those two features to use the same add_lookup
function as pluggable InputTypes.

[LMS-2489]

Quick and dirty fix (edxapp.startup.run is called twice, once before
loading the configuration, so the paths defined in lms.startup are
basically ignored).

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__)