Add bin and course validations to user.

And Add move_course, but don't tell anyone.
Added API endpoint to add new course to new bin at end of bins
Added API endpoint for adding a course to a specific bin.

Also refactored some code in preparation for adding bins to a specific place.
That's the next step.
WIP: :shit: embed bins within users for reordering.
I think create_bin_before works
:fire: change method names to snake_case, and refactor if/else
Merge branch 'bins'