Admin console function to send test email / verify smtp setup [customer]

We have had some trouble with our smtp-setup, and miss a way to easily test the smtp setup from the admin console.

Many / most other similar applications that have an email-feature (think Atlassian-products, jenkins, sonar, artifactory...) have an easy to use "test email" feature from their admin console , which allows administrators an easy way to generate a test email. GitLab should have the same.

gitlab-ce#15460 talks about a similar feature for command line usage (a rake task). We'd prefer an option in the admin console, in part because not everybody has access to the linux server console.