Bug 31374: Update Suggestion.pm

To test:
1. Apply patch and restart services
2. Go to Acquistions -> Suggestions -> New Purchase suggestion
3. Notice the new 'Non-public notes:' textarea
4. Make sure you 'Set a patron' so we can later look at the suggestion from the OPAC
5. Make a new suggestion and make sure you add some Non-public notes
6. Back on the 'Suggestions management' page you should see a new column for 'Non-public note'
7. Go to table settings and make sure you can properly hide the column
8. Change the status and make sure it shows up on the table with all statuses (pending, accepted, rejected, etc )
9. Check the API by going to (/api/v1/suggestions). Your suggestion should show up and 'privatenote' should include your non-public note.
10. Log in to the OPAC as the patron we set the suggestion to earlier. You should not see anything related to the non-public note displaying from the OPAC

Signed-off-by: David Nind <david@davidnind.com>

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

Koha/Suggestion.pm

@@ -160,6 +160,7 @@ sub to_api_mapping {
         lastmodificationby   => 'last_status_change_by',
         STATUS               => 'status',
         note                 => 'note',
+        privatenote          => 'privatenote',
         author               => 'author',
         title                => 'title',
         copyrightdate        => 'copyright_date',