Add mail-debug flag to rest-server

It has been added to the joex application, but it should be possible
to debug mail problems on both apps.

modules/restserver/src/main/resources/reference.conf

@@ -79,6 +79,11 @@ docspell.server {
 
   # Configuration for the backend.
   backend {
+    # Enable or disable debugging for e-mail related functionality. This
+    # applies to both sending and receiving mails. For security reasons
+    # logging is not very extensive on authentication failures. Setting
+    # this to true, results in a lot of data printed to stdout.
+    mail-debug = false
 
     # The database connection.
     #