mailers: prevent the signature from being auto-linked

Some emails clients (Gmail or Mail.app) may turn the signature into
a clickable link.

This can distract users, and make them think we are a good point of
contact (where they should contact their administration or use the
website directly instead).

app/views/layouts/mailers/_signature.html.haml

@@ -4,4 +4,5 @@
   - if defined?(service) && service && service.nom.present?
     = service.nom
   - else
-    L’équipe demarches-simplifiees.fr
+    -# The WORD JOINER unicode entity prevents email clients from auto-linking the signature
+    L’équipe demarches-simplifiees⁠.fr