lib/user.php: new institutional users should have license set to "institution default"
Bug #1162134. Previously we were storing the "institution default" value as "-" in the database. I've changed this to saving a constant, LICENSE_INSTITUTION_DEFAULT, which is defined as "(institution default)". Change-Id: I05337d9e8b1b547f12e4658b8d78fcbe1b5b2ea4