unenrol instructors even if it is their last section
add site identifier to outgoing UES email messages in order to reduce confusion. See https://trello.com/c/axm1bacF
udpate comment and remove a reference to  that I missed earlier.
update default visibility assignment to rely on sitewide default
Merge pull request #41 from lsuits/unwant

unenrol instructors even if it is their last section