Add Server base URL to 'Test Configuration' email

When sending out a test e-mail from my SonarQube instance, there are two things I want to check

1. That my SMTP configuration is correct and an e-mail sends at all
2. That the Instance URL is configured correctly and not falling back to localhost:9000, or that I have a bad value in sonar.core.serverBaseURL (a URL that I can test is correct).

We currently tick the first box. We should add the Server base URL to the email content to tick the second one.