Bug 9372: replace carriage return with <br /> in printed HTML files for overdues

For PDF generation, HTML files need explicit <br /> HTML tags. With
this patch, carriage return are replaced by <br /> when letters are
written into HTML files.

Test plan :
- Ensure you've got at least one reader with overdue triggering notice
- Ensure that your notice template got HTML checked
- Run misc/cronjobs/overdue_notices.pl -v -n -html <dirname>
  and misc/cronjobs/printoverdues.sh <dirname>

Without patch, the content of the letter is all on one line in PDF file
With the patch, PDF file is correctly formated

- remove your reader email
- Run misc/cronjobs/overdue_notices.pl -v -html <dirname>
  and misc/cronjobs/printoverdues.sh <dirname>

Without patch, the content of PDF file is all on one line
with the patch, PDF file is correctly formated.

- restore reader email
- run misc/cronjobs/overdue_notice.pl -v -html <dirname>

Verify that the a message has been generated in message_queue table
With or without patch, the message is the same

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Bug 9372: (follow-up) tidy whitespace in new lines

Signed-off-by: Galen Charlton <gmc@esilibrary.com>