SAK-23852 modify external subscription field display name

git-svn-id: https://source.sakaiproject.org/svn/calendar/trunk@132146 66ffb92e-73f9-0310-93c1-f5514f145a0a
SAK-23851 validate the url


git-svn-id: https://source.sakaiproject.org/svn/calendar/trunk@132147 66ffb92e-73f9-0310-93c1-f5514f145a0a