Added Associations & Validations

-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

**works on my end but if there are any issues put this in:  <%= javascript_include_tag 'application', 'data-turbolinks-track' => true %>   at:  app/views/layouts/public.html.erb

**Still working on deleting the names table
Please clean up indentation on these, too.