Include use counts for additional options in main admin view

This adds some extra work to viewing the list, but the dataset is
small enough that it should not have any real impact on performance,
and it's useful for a birds-eye view of AO usage.
Make it possible to search for users when creating vouchers

The dropdown of users is getting very large. Tihs commit copies over
and slightly modifies the code from the invoice creation system. It
could probably be moduralized as we can use it in many places, but
that's for a future commit.
Allow auto-generation of invoices for pre-paid vouchers

Will generate an unfinished invoice as we don't have all the data,
and doesn't connect them together, but removes a lot of copy/paste
work.
Show voucher usage statistics in main admin interface
Don't show email about prepaid batches to end-users
Make it possible to directly email generated prepaid vouchers
Include link to overview page in prepaid voucher email
BCC invoice emails to treasurer@

This will BCC all manual invoice emails only. This is basically
all the emails generated except receipts that are generated for
invoices that have a processor set.