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).

The difference is that apply_preferences() deletes all of the user's optional line items for the program, whereas set_preferences() deletes only those of a certain type.  However, right now set_preferences() is defined very narrowly (that's why there is all that code figuring out which Transfers to delete); it only changes the quantity of line items that otherwise match exactly.

I could try to figure out a way to merge them, but how about just removing set_preference()?  Right now it's only used by SplashInfo.save().