Bug 12123: Prevent HTML notices to break the notice viewer

Bug 12123: Prevent HTML notices to break the notice viewer

Depending on the content of an html notice, it can cause the notice to
either not be collapsible, be uncollapsible, or to be permanently
collapsed.

Test Plan:
1) Set your CHECKOUT notice to the following ( with HTML Message checked ) :
<p>The following items have been checked out:</p>

----
<blockquote> <<biblio.title>> </blockquote>
----

<p>Thank you for visiting the <<branches.branchname>> of HMCPL.</p>
2) Check out some items to a patron
3) View the patrn's notices
4) Note the notice viewer is broken ( message is not collapsed, and
   con't be collapsed ).
5) Apply this patch
6) Reload the page
7) Note the notice viewer is no longer broken

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
I followed the test plan successfully.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>