Added Associations/Validations for Cohort, Event, Notification and User

-Cohort: added assignment association
-Event: added validated name, start/end dates
-Notification: added validation for user, message, sender email/name
-User - added association for attendance, announcements and notifications
This looks not right to me.
Doesnt it need to validate cohort and class as well?
That or it doesnt need to validate starts_at and ends_at?
Someone take a look.