Bug 31097: (follow-up) Restructure messages a bit

Bug 31097: (follow-up) Restructure messages a bit

This patch updates the patron restrictions markup and splits up some
strings to (hopefully) improve the translation process. The patch also
fixes a bug in a previous patch which prevented manual restrictions from
being displayed if there was also a restriction added by
overdue_notices.pl.

Test that all three kinds of restrictions display correctly:

 - Restrictions added by overdue process
 - Manual restriction without an expiration date
 - Manual restriction with an expiration date

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