Change Stripe module from abstract to proxy
Conditionally show link to confirmation on payment success page

If students aren't able to confirm their registration (because the
deadline is closed), we don't want to link to it and confuse them with an
error.
Change e-mail link on payment page to program directors

The webmasters are not the right group to e-mail (and with recent changes
to our support structure, the e-mail would go to a chapter internal list
anyway).
Force students to complete required modules before paying
Make registration receipt open in new tab