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 accounting controller is mostly or entirely fine if we were to implement that. There are a few places in the module that wouldn't work. This particular line, for example. Because the keys for this dictionary are line item types, which means you can only have one selection per line item type.

This is fine for now, we don't need to make any changes. But this would need to change in the future, if we implemented checkboxes/quantities for line item types with options.