Bug 22888: Use DataTables for Koha news table filtering

Bug 22888: Use DataTables for Koha news table filtering

This patch modifies the Koha news page so that filtering the table of
news is done using DataTables' built-in search functionality. This
allows for the table to be filtered without requiring a reload of the
page.

The patch also moves the table filter into a sidebar form and adds a
keyword field. A minor change has been made to the global CSS to improve
the display of the form in the sidebar.

To test, apply the patch and rebuild the staff client CSS.

 - Go to Tools -> News.
 - Test the various table filter options: keyword, display location, and
   library. Confirm that all work as expected.
   - Changes to the keyword search text should be reflected in the
     search field at the top of the table, and vice versa.
 - Changes to the sidebar filter should trigger the correct state of the
   "Clear filter" button at the top of the news table (enabled or
   disabled).

Signed-off-by: Maryse Simard <maryse.simard@inlibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>