Fix associating an assignment with a course from the assignment.
[E1452] Refactoring: renaming
 - Replaced 'get_participants' method with duplicate 'participants' method
   and removed 'get_participants' in AssignmentTeam model.
 - Renamed the 'create_team_and_node' method to 'create' to better follow
   Ruby conventions.
 - Renamed 'remove_team_by_id(id)' to 'delete(id)'.
 - Renamed the 'includes?' method to 'has_participant?'.

Signed-off-by: Spenser James <sjames3@ncsu.edu>
[E1452] More invasive refactoring.
 - Renamed usages of team 'member' to 'participant' to be more consistent.
 - Re-ordered and re-organized code so that it flows more logically as it
   is read.
 - Refactored some logic control statements so that they are easier to read
   (replacing !(<logic>) with unless, etc.)

Signed-off-by: Spenser James <sjames3@ncsu.edu>