Fixes #31: Introduced a better way to preview and test campaigns …

[david-smejkal]

… using a configurable (optional) Test Contact

• [x] Alleviates the necessity to have a contact record in the database for some of Birdsong's functionality to work out-of-the-box.
• [x] Prevents accidental unsubscribing of the first contact in the database when using unsubscribe links in previews.
• [x] Works with Send Test functionality.
• [x] Works without any modifications even in projects with Extended Contact models.
• [x] Introduces a new optional BIRDSONG_TEST_CONTACT setting that can be used to further extend and control the preview / send test experience.