Update for 5.0 branch to teachPress 5.0alpha18. This is a sync with GitHub repository: https://github.com/winkm89/teachPress/tree/fc081c0d3876a62c3a5703a4ca0e2356843cb20b

git-svn-id: https://plugins.svn.wordpress.org/teachpress/branches/5.0.x@1036865 b8457f37-d9ea-0310-8a92-e5e31aec5664