Course Pages Redesign and Features

Many changes here due needing to change other files so they would conform with the changes here. 

* Course overview, creation, and index are completely redesigned
* Course overview has new enroll button and includes a description if the user provides it
* Course overview has buttons to handle editing and deleting -- no implementation yet https://github.com/Eduki/eduki/issues/99
* Users can now **unenroll** from a course https://github.com/Eduki/eduki/issues/92
* Course creation handles a description. This is **required** for a course, even though the database has the description has optional
* Conditional URLs for ajax requests. See: /models/course.js or /models/enrollments.js (handles delete and create)
* Tests