Update for 5.0 branch (add basics for document management, new shortcode [tpcoursedocs], rewritten show_courses_page, hundreds of small changes). This is teachPress 5.0alpha6.

git-svn-id: https://plugins.svn.wordpress.org/teachpress/branches/5.0.x@964811 b8457f37-d9ea-0310-8a92-e5e31aec5664