Bug 34728: Make pre tag conditional

Bug 34728: Make pre tag conditional

This patch makes the pre-formatting of notice templates conditional on the content type.

Test plan:
1) Navigate to tools > Notices and slips
2) Edit the notice with the code of HOLD
3) Under print, paste the following
<p>Please collect within 10 days from the date of this letter and don't forget to bring your library card with you.</p>
<p>Be aware that opening hours vary at different library branches so if in doubt double check the times before you visit.</p>
<p>Should you no longer require this item please contact us so we can make it available for another customer.</p>

<p>Thank you</p>
4) Check the box that says HTML message and save the notice
5) In Patrons, select a patron and in Patron messaging preferences, check the box in the Email column for Hold filled
6) Find a record in the catalogue
7) Copy the barcode for one of the items
8) Place a hold on the item for the patron you selected
9) Navigate to Circulation > Check-in
10) Paste the barcode in the Check-in box and click Check-in
11) In the pop-up box, click Confirm hold
12) In the command line, run perl misc/cronjobs/gather_print_notices.pl --letter_code=HOLD --html /tmp
13) cd /tmp
14) There should be a file in there called 'notices_HOLD-date.html
15) cat this file
16) The html you pasted in the template will be wrapped in a <pre> tag
17) Apply patch and restart_all
18) rm the notices file that was created
19) cd /kohadevbox/koha
20) Select a new record from the catalogue and repeat steps 7-15 for this record, using the same patron
21) This time, the html file should not contain the <pre> tag
22) Sign off!

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>