Bug 36233: Use select2 to load vendors on invoice search

Bug 36233: Use select2 to load vendors on invoice search

This patch moves the dropdown to use select2 and avoids loading all
vendors at page load.

To test:
 1 - Create some extra vendors in your system, ideally over 20
 2 - Search for a vendor in acquisitions
 3 - Click 'Invoices'
 4 - Note the dropdown of all vendors 'Vendor:' in search bar on left
 5 - The vendor you came from shoudl eb selected
 6 - Apply patch
 7 - Repeat
 8 - Note only a partial list of vendors is loaded, confirm current vendor still selected
 9 - Search in the dropdown and confirm vendors are returned
10 - Select a vendor and search
11 - confirm selection is retained

Signed-off-by: Lisette Scheer <lisette@bywatersolutions.com>
Rebased-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>