Consider making crsid field compulsory for postgrads

We seem to get a few every year who don't put their crsid onto the form, don't get matched to their pre-existing record (because matching by surname isn't as reliable) and end up with duplicates in the database that have to be merged by hand. I don't think it's possible for them /not/ to have crsids if they've been admitted as students, unlike other categories, so we could just require postgrads to enter the crsid.