Bug 26501: Add datatables to article-requests form

Bug 26501: Add datatables to article-requests form

Actually two tables: one for pending and processing.
Variable active_table has been renamed to active_tab.
To prevent a flickering form we hide stuff until ready.

Test plan:
[1] Submit a few article requests.
[2] Verify that Process, Complete and Cancel still work via Actions.
[3] Test again with multiselect and top Actions menu.
[4] Test Search and Sort on the datatable.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>