Add initial support for non-EUR currencies

Put the EUR abbreviation and the € symbol as settings in settings.py
instead, with the default obviously being Euro. We still only support
one currency per installation configured globally, to keep things simple.

The paypal integration requries a separate setting for currency, as
some of that code is not in django (yet).

This is intended in particular to be able to run the confreg system
for non-euro conferences, but the settnigs are made global for the site.
Remove old invoice managers ability to create invoices

This hasn't been used for over a year, and started causing import
errors. We still leave the ability to *view* old invoices around,
which is the only reason we kept the code at all.
Make the invoice builder configurable

Since we already have a single file with the code that builds the complete
invoice, just rename it to a different name (pgeu specific), and make it
possible to specify a completely different one in the config file.

We still only support one invoice generator per installation.