Re-add and apply MDL rule MD040

We were following it about half of the time and we even added it to our
former `.mdlrc` file. However, for some reason, MDL doesn't detect this
rule when specified in the `.mdlrc` file, so we didn't notice we weren't
following it in many cases.

Now that we're using a style file to configure MDL, we can enable this
rule again and apply it, since now MDL correctly includes it in its
report.

docs/en/installation/deploying-on-heroku.md

@@ -113,7 +113,7 @@ Add the SendGrid add-on in Heroku. It will create a SendGrid account for you wit
 
 Add this to `config/secrets.yml`, under the `production:` section:
 
-```
+```yaml
   mailer_delivery_method: :smtp
   smtp_settings:
     :address: "smtp.sendgrid.net"