Change extracost selections after paying

[willgearty]

This adds the ability (enabled by default) for students to return to the extracosts page after they have paid via credit card and choose different extracost options. This could be a 1 for 1 change (where there is no balance change), a positive change in balance (where the student would then need to pay via credit card again), or a negative change in balance (which we treat as a temporary/permanent donation).

The students can return to this page as many times as they would like between and after paying (provided the tag is enabled, which is the default). As changes are made, the (new!) subtotal (and balance if the student has paid already) at the bottom of the form updates automatically.

Fixes #3570.

[willgearty]

I've made the following changes:

• Financial aid now updates dynamically in the extra costs form
• Reordered the subtotal lines
• More help text for the line items form to try to prevent cases of financial aid covering lots of quantities of items