Bug 30287: Conditionally render notice preview

Bug 30287: Conditionally render notice preview

This patch adds conditional formatting based on whether the notice was
generated from an HTML template or a plaintext one. We simply drop the
'html_line_break' filter for the case where the notice should already be
html formatted.

I was tempted to replace the html_line_break for plaintext formatted
notices too as I believe this closer resembles how we actually output
such generated notices.. but I decided that should be for another bug.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>