Clean and refactor courseware/views.py and module_render.py

* Refactor index() so that it makes sense to me and hopefully others :)
* Rename preloaded cache of student modules to student_module_cache
* Fix line length and whitespace throughout
* add docstrings and other comments
* a few behavior-preserving tweaks to the code to make it clearer.
* Separate codepaths for with-module and without-module in index view
* Remove default chapter + section, since they don't exist anyway in course.xml