Fix: Current Bills thinks it is Bill History

Fix: Current Bills thinks it is Bill History

This fixes LP 855894: Bills interface calls itself Bill History,
uses bills_historical receipt template

https://bugs.launchpad.net/evergreen/+bug/855894

At some point in development, the Current Bills and the Bill History
interface were in a unified file, with an xul_param of "current" used
to switch between the two.

Nothing currently sets the xul_param "current", so the Current Bills
interface (bill2.xul and bill2.js) was improperly:

* not hiding the xact_finish column
* using a caption label of "Bill History"
* using the bills_historical receipt template

The Bill History interface was okay -- "not current" was the default.

This commit removes the tests for xul_param('current') and uses the
appropriate code for each interface.

// this code in both bill2.js and bill_history.js:
var template = 'bills_historical';
if (xul_param('current')) template = 'bills_current';

// becomes:
var template = 'bills_current';    // in bill2.js
var template = 'bills_historical'; // in bill_history.js

Also: added missing staff.patron.bill_interface.caption.label entity

Signed-off-by: Jeff Godin <jgodin@tadl.org>