Add course model and associations.
Add course API.
Update UI views to reflect new course scope API.

Note the lessons are now referenced by their BSON ids.
Make top home page link more noticably clickable.
Add lead_teacher relationship.

This is needed to decide which user (Teacher) will be responsible for providing files to the course's lessons.

Each lesson will query a scoped Dropbox endpoint on the lead_teacher's account.
Provide materials API (file system) via Dropbox.
Add materials UI react component.
Require lead_teacher on course.
Autoload the first tab on lessons view.
Add default link styles.
Materials 'download all' link is now button.
Add https://getsentry.com/
Add ability to grant access to courses for students via access_code.
Simplify the teacher <-> course relationship.
assets