rewrite template tags

[nikolas]

Rewrite the template tags in templatetags/coursetags.py. The implementation is using outdated django templatetag style. In addition, it also requires complex syntax in the template to call it, like this:

{% course_role for request.user in course as role_in_course %}

I find this a little confusing. Is that a for loop, or what? See if we can clean up this syntax.