LP#1745232 - Bill History Receipt doesn't have Finish Date

LP#1745232 - Bill History Receipt doesn't have Finish Date

Adds xact.finish and xact.summary.last_payment to the example data.  Also adds xact.summary.last_payment_ts to the template body to indicate when the last payment was made.  Also adds a list of variables that can be used in the template in the template's description.

To test -
1. apply the patch
2. find a patron with billings, or add some billings to a patron record.
3. make payments on some of the billings using various methods.
4. go to billing history and print the history list.

Signed-off-by: Garry Collum <gcollum@gmail.com>