Check donation module when filtering line item types for extra costs

We were doing this for the Stripe module, but now that the donation module
also exists, we have to check if it is being used also.
Adjust default link_title for donation module
Use separate setting tag for donation module

People might be confused by needing the "stripe_settings" Tag if they
aren't using Stripe.

Also, removes the "offer_donation" setting; if you're using this module,
it wouldn't make sense to have offer_donation = False.
Add line_item_type() method to DonationModule
Update students() and studentDesc() in DonationModule
Change deadline for donations to /ExtraCosts

We could make a separate deadline if this isn't sufficient.
Take out some unused code in donation module
Fix typo causing donation choice to be 'stuck'
Hide donation choices until user checks the box
Tweak text in donation module template