Create 3 new milestones for every newly created course ...

Three seems sufficient given that a single course will have
10-12 lessons and 10-12 quizzes => 4 lessons/quizzes per milestone
No need for after_create callback in Milestones ..

Milestones are created when the parent course is created.
Moreover, a fixed number (3) is created. Hence, no need
to have intelligence within the milestone for what its index
should be
Minor stylistic change
Corrected association specifications ...
Corrected association specifications
[single-line clicks]: Processed within the same event handler ...

Clicks on a menu-item in a menu within a single-line should be handled
within the single-line event handler

Previously, all single-line clicks - except one, on menu-items - was handled
by one event handler and the remaining one was handled by another
event handler.

This latter event handler still exists - but is limited to
handling menus within control-panel and toolbox
Added some useful query and utility methods ..
Removed duplicate definitions of methods auto-added by Rails due to associations
Rationalized handling of videos ...

Videos are now rendered as single-lines .. And they can have
contextual menus like single-lines do.

Important changes made in core/trigger.js.coffee to handle clicks
on menu items in .single-line > .menu