Bug 21216: (follow-up) Exclude some columns from export

Bug 21216: (follow-up) Exclude some columns from export

This patch adds a custom buttons configuration to the Notices table so
that columns with form controls are not included in copy/print/export
operations.

The DataTables columns settings include file has been modified
to check for an existing buttons export columns configuration before
applying the default.

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

Go to Tools -> Notices & slips. The table of notices should have a
DataTables toolbar with a filter form, column visibility, and export
buttons. Test the functionality of all these tools. Only the first four
columns of the table should be included in printouts and exported files.

Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>