Bug 31028: (follow-up) Consistently style help-block

Bug 31028: (follow-up) Consistently style help-block

This patch updates the opac and staff modals to set the help-block
inside a div instead of a paragraph element allowing for the wysiwyg
edited content to display as prescribed.

We move the scss inside the fieldset definition to ensure we are
specific enough to catch only the intended elements.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Helen Oliver <HOliver@tavi-port.ac.uk>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>