Allow admins to assign default sticky colors to participants when they create/send invites

Allow admins to assign default sticky colors to participants when they create/send invites, and also provide an option for the system to just automatically assign a unique color/style for post-its  for each participant that enters a session.  This will make it infinitely easier to differentiate post-its during a session and will cut down on a lot of manual work from facilitators when creating / setting up a session.