LP#2036842: new reporting sources for invoice totals

LP#2036842: new reporting sources for invoice totals

This patch adds three new reporting sources to permit reporting
on the total amount of invoices, including both line items and
direct charges. The sources are:

  * Invoice Totals
  * Invoice Debits by Fund
  * Invoice Debits by Fund Tag

These sources are meant to be used as the base source of a report,
with any additional fields and tables of interest brought via
navigating links in the template editor.

To test
-------
[1] Create some invoices that include line items and direct charges
    with associated fund debits. Also create and assign some fund
    tags.
[2] Apply the patch.
[3] Create reports that use the new sources and verify that the invoice
    totals, totals by fund, and totals by fund tag are correct.

Sponsored-by: Evergreen Indiana
Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>
Signed-off-by: Beth Willis <willis@noblenet.org>
Signed-off-by: Jane Sandberg <js7389@princeton.edu>