Add missing mimetypes for xblock resources

This PR is related is related to https://github.com/edx/XBlock/pull/214 . It adds the missing font mimetypes for the xblock resource rendering. Without this patch, django simply crash with a DjangoUnicodeError. The mimetype is used here:

https://github.com/aboudreault/edx-platform/blob/e011eb1ab5defc3af80e5019b43b48b32d8a6bb2/lms/djangoapps/courseware/module_render.py#L648

CC, @antoviaque, @jimabramson