Handle student & coach event emailing as separate

Send them both in the background, separately. This should help with
the timeouts we have sometimes on Heroku.

app/models/invitation_manager.rb

@@ -33,13 +33,15 @@ def invite_students_to_event(event, chapter)
       EventInvitationMailer.invite_student(event, student, invitation).deliver_now if invitation.save
     end
   end
+  handle_asynchronously :invite_students_to_event
 
   def invite_coaches_to_event(event, chapter)
     chapter_coaches(chapter).each do |coach|
       invitation = Invitation.new(event: event, member: coach, role: 'Coach')
       EventInvitationMailer.invite_coach(event, coach, invitation).deliver_now if invitation.save
     end
   end
+  handle_asynchronously :invite_coaches_to_event
 
   def chapter_students(chapter)
     Member.in_group(chapter.groups.students)