Refactored course_nav middleware to isolate the django-wiki hack, remove unnecessary code, and clarify what it's doing.

This PR removes the middleware and context processor that were in course_nav.py. The middleware isn't required because it only affects the course wiki and root wiki pages. Instead, we can wrap the django-wiki functions in a view function that accomplishes the same tasks. Similarly, the context processor that used to load the course based on a regex of the current url can be loaded explicitly in the wiki template, rather than included on every page.

@ormsbee 
@chrisndodge regarding https://github.com/edx/edx-platform/pull/2713 this is the refactor I mentioned :)