Bug 35285: Add non-html template support to html_content wrapping

Bug 35285: Add non-html template support to html_content wrapping

This patch adds support for messages generated using non-html formatted
notice templates to the html_content method of Koha::Notice::Message.

We continue to wrap content for html generated messages with the
appropriate headers, css and title.

For non-html generated content we wrap in the <div style="white-space:
pre-wrap"> block to maintain text formatting as defined in the original
plaintext template.

Test
Follow the test plan for bug 30287, nothing should outwardly change.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>