Make the email templates tags table occupy the whole width available

It improves the content formatting (no more ugly
line breaks splitting a tag on several lines)

app/views/admin/mail_templates/edit.html.haml

@@ -15,10 +15,10 @@
       = f.button :submit, 'Mettre à jour', class: "btn-success"
 
   .row
-    .col-md-8
+    .col-md-12
       %table.table
         %tr
-          %th
+          %th.col-md-3
             Balise
           %th
             Description