Bug 26189: Improve styling of datatables within fieldsets

Bug 26189: Improve styling of datatables within fieldsets

Unset label width restriction: The label width restriction within
fieldsets causes issues for the datatables header. Unsetting it
explicitly here resolves the line wrapping issues :)

Reset margins for datatables found inside fieldsets such that the
datatable properly utilises the available space and restore the
default datatable label font weights.

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>