Bug 11607: Don't display all items in a single line

Bug 11607: Don't display all items in a single line

If the *DGST notices are sent in HTML, the items are displayed in a
single line.

To reproduce:
1/ Define a *DGST notice using the <<items.content>> pattern.
2/ Checkout at least 2 items to a patron and set the due date as today.
3/ Launch the advance_notices.pl and process_message_queue.pl cronjobs.
4/ Verify the email you will receive separates the items with a line
break.

Verify you don't find a regression for non-html letters.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Note: the display in the notices tab is misleading here,
needs to be verified checking the sent emails or database
entries in message_queue.
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>