New class grad import improvements

[dehnert]

Some deficiencies found in this semester's import:

• The DB does not support marking that somebody is a (non-MIT) student in the import spreadsheet. It probably should, because I assume handling that data in spreadsheet form is more pleasant than clicking on a bunch of folks.
• It also doesn't support automatically giving folks a subscription for the matching class (we should probably actually track the subscription period <-> class mapping in the DB persistently, rather than throwing this info away)
• We mark people as graduated from the class. If we want to eventually import people earlier in the class, and then mark them as graduated later, we should:
  • allow specifying the initial membership status,
  • provide a convenient "change membership status" view, and
  • notice when people have an existing person record