Add support for line item options

This adds a foreign key from Transfer to LineItemOptions (as suggested by Jordan in #1119) and updates the accounting controller and StudentExtraCosts module to make use of it.  You can test this by creating new line item types for a program; you can add options (each with a description and optionally a new cost value) from the admin page for the line item type.  You should see each of these options on the extra costs page in student reg, and it should remember your choices when you save and come back later.

Addresses #1119 and #1120.  However, it could be argued that more front-end work is desirable for #1120.  Also, we don't support a combination of multiple LineItemOptions choices and variable quantity (in the StudentExtraCosts module; the accounting controller should be able to handle it).

I see the difference in functionality and use-case now. I think it's fine the way it is, though in the future we can reduce code by making ```apply_preferences()``` call perform the same delete it is doing now and then call ```set_preference()``` a bunch of times.

It now makes sense to me to have ```set_preference()``` not delete everything, so we shouldn't remove it.