Bug 26745: Add TT support to 'title' in notices

This patch adds support for using TT syntax in the title element of the
notices system.

Test plan
1/ Edit an easily triggerable email notice such that the title contains
   some template toolkit markup.
   Example: Edit the 'CHECKOUT' notice to include [% today %] in the
   title field.
2/ Trigger the notice (For 'CHECKOUT' that means checking an item out to
   a user (Preferably with 'email' selected in their advanced messaging
   preferences for the notice)
3/ Either, navigate to the 'Notices' tab for the user, or run the
   message queue and inspect the subject line of the recieved email.
4/ Prior to the patch the TT syntax would remain 'as is'.. i.e
   '[% today %]'.
5/ After the patch, the TT syntax should have been properly replaced.
6/ Signoff

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

C4/Letters.pm

@@ -708,6 +708,15 @@ sub GetPreparedLetter {
         }
     );
 
+    $letter->{title} = _process_tt(
+        {
+            content => $letter->{title},
+            tables  => $tables,
+            loops  => $loops,
+            substitute => $substitute,
+        }
+    );
+
     $letter->{content} =~ s/<<\S*>>//go; #remove any stragglers
 
     return $letter;