Bug 11217: The # in accountlines descriptions makes them un-writeoffable

Bug 11217: The # in accountlines descriptions makes them un-writeoffable

This patch adds escaping to url params in mambers/pay.pl

To test:
- Apply patch
- Go to Home > Patrons > Manual invoice and create an invoice with a
  description containing a #
- Go to tab "Pay fines"
- Write off the fine
Expected result: Fine is written off and does no longer display in pay tab.

Signed-off-by: Claire Gravely <claire_gravely@hotmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>