lib: Added new package pulldown_subj, a collection of modules designed to pull student subject enrolments from the server. Note that the actual code to do this is not included (since that is specific to the organisation running IVLE) - just a pluggable interface and an example plugin module. configure.py: Added new config option: subject_pulldown_modules, which allows you to specify which modules are plugged in here. (Actually that was added accidentally in a previous commit; but this revision fixes some comments).