site permissions: add class list by section number, or by external group code

site permissions: add class list by section number, or by external group code

If you provide a hook function, 'derive_group_code_from_section',
mapping section numbers onto external group/class-list codes (in the
context of a given site), then users can easily add sections to course
sites, just by providing the section number(s). If not, then the part
of the UI related to section numbers is not displayed.

Note that external group members are loaded lazily: we don't know who
belongs to an external group until they log into Syrup, and their
external memberships are updated.

git-svn-id: svn://svn.open-ils.org/ILS-Contrib/servres/trunk@1122 6d9bc8c9-1ec2-4278-b937-99fde70a366f