Bug 21814: (follow-up) System preferences save button can be hidden by language menu

Bug 21814: (follow-up) System preferences save button can be hidden by language menu

This follow-up implements the same change by defining a variable to be
used as the minimum height of the language footer navbar. A calculation
is done on this value to set the bottom margin of the main area of the
page.

To test, apply the patch and rebuild the staff client CSS. Clear your
browser cache if necessary. View the system preferences page again and
confirm that the bug is still fixed.

Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>