Bug 22175: (follow-up) Identify columns to exclude from export

Bug 22175: (follow-up) Identify columns to exclude from export

Bug 21216 added a way to designate DataTables columns to include in
export operations (Copy, CSV, print, etc). However, this solution did
not take hidden columns into account. This patch revises the global
columns configuration settings so that exports will include only visible
columns and will exclude columns with a "noExport" class on the <th>.

To test, apply the patch and clear your browser cache if necessary.

 - Go to Tools -> Stock rotation
 - In the table of rotas, hide some columns in the table and verify that
   export operations include only your visible selections, excluding the
   last column with buttons.
 - Choose a rota and select Manage -> Items. Test that this table works
   in the same way.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>