Bug 13040 - multiple branch selections in exporter

Bug 13040 - multiple branch selections in exporter

This allows the exporter (Tools -> Export) to have any combination of
branches selected, rather than it being all or only one.

Test Plan:
* Apply the patch
* Go to the exporter, see that instead of a dropdown you now have an
  elegently laid out grid of branches you can select from
* Select some branches, run the export
* Note that only records with items in the selected branches are
  returned.
* Repeat this with the item related options (as that code was refactored
  slightly) and make sure everything is sane.

Sponsored-By: South Taranaki District Libraries
Signed-off-by: Thomas <tomsStudy@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>