Styling for a new class - .light - defined
New join-table - Progression - added ...

This table tracks the progress of students through milestones.
Going forward, we might decide to implement a scheme in which the
next milestone is opened only once the student has completed the
current milestone
.tabs-left and .tabs-right treated the same way
[Lesson]: Renamed column 'title' to 'name' to stay consistent with other models
Two simple query methods - lessons and quizzes - added
Two new query methods - available_[lessons | quizzes] - added ...

These return the lessons and quizzes that aren't yet part of
the given course
Changed all old-style :via => sth to via: sth
Initial framework for listing Course outline and available assets

Next step: Attaching / detaching assets from selected course