Bug 32085: Consistent classes for primary buttons: Labels

Bug 32085: Consistent classes for primary buttons: Labels

This patch makes changes the button markup in label-creation templates
-- including the barcode image generator -- so that all submit buttons
and any buttons that should should be styled as primary buttons have the
Bootstrap class "btn btn-primary."

To test, apply the patch and view label-related pages to confirm
that everything looks correct. In most cases there are no visible
changes.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>