Bug 27884: Add html support for report mails

Bug 27884: Add html support for report mails

To test:
1) Create a new notice, for example with Koha module "Patrons",
   name/code TEST and message body "<strong>testing<strong>".
2) Create a new sql report, the query could be someting like:
   SELECT "<number>" as `borrowernumber`, "to@example.com", as `email`, "from@example.com" as `from`;
   where "<number>" is a valid borrowernumber.
3) Run patron_emailer.pl --report=<id> --notice=TEST --module=members -commit
   where <id> is the report id.
4) Check the message_queue table that the content_type column has been
   set to text/html; charset="UTF-8".
5) Ideally process the message queue and veriy that the sent email is displayed
   as rendered html.
6) Run tests in t/db_dependent/Reports/Guided.t

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>