Bug 22015: Move DataTables CSS to global include

Bug 22015: Move DataTables CSS to global include

DataTables are used on enough pages in the staff client that it
doesn't make sense to put inclusion of the CSS into each template
where it is needed. This patch moves includes of datatables.css from
individual templates into the global header file.

To test, apply the patch and view various pages which have DataTables.
View various styles of DataTables, e.g.

 - Full pagination, like item search results
 - Four-button, like Saved SQL reports

Everything should look the same as it was.

Signed-off-by: Jose-Mario Monteiro-Santos <jose-mario.monteiro-santos@inLibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>