Bug 10737: Add "Add to basket" link on vendor search results page

Bug 10737: Add "Add to basket" link on vendor search results page

This patch adds an "add to basket" link to the vendor search results
page for each open basket associated with each vendor. Clicking it
triggers a modal window with the "add to basket" choices for that vendor
and basket.

Other changes in this patch:

- The add-to-basket include has been modified in order to make it more
  useful in this context.
- booksellers.pl has been modified to check for an existing budget so
  that the add-to-basket include can properly display a warning if there
  are none.
- "New basket" and "Receive shipment" buttons associated with each
  vendor search result have been converted to Bootstrap-styled buttons.
- Basket closed date has been moved into its own column so that the
  table can be sorted by that value.
- Table columns containing dates now use the "title-string" sort option,
  eliminating the need for a special date sorting algorithm.
- Converted some &'s to &'s

To test, apply the patch and search for a vendor. For each vendor in
your search results baskets which are open should include an "add to
basket" link. Clicking it should open a modal dialog with the same "add
to basket" options offered on the basket page. The correct vendor ID and
basket number should be associated with each link.

The newly-styled "new basket" and "receive shipment" buttons should work
correctly. Table sorting should work correctly, including the new
"closed" column.

Since the add-to-basket include file was modified, the "add to basket"
button on the basket view page should also be tested (acqui/basket.pl).

Signed-off-by: Campbell Reid-Tait <campbellreidtait@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>