Bug 30287: (follow-up) Add content wrap matching send time

Bug 30287: (follow-up) Add content wrap matching send time

This patch adds a new method to the Koha::Notice::Message class to wrap
content in the same way we currently do at email send time. We then
update the notice preview to use this method so we're embedding
NoticeCSS into that iframe. This means the generated preview should now
exactly match how your notice will display in email clients including
any CSS you may have placed within NoticeCSS.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Fixed typo in POD of html_content

Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>