Mailgun improved endpoint support

Use services.mailgun.endpoint as base URL for Mailgun API, to make the endpoint configurable for both US as EU endpoints.

Added full Mailgun support, unsubscribe event & removing SDK dependencies

• Removed SDK dependencies for Postmark and Mailgun
• Added config 'mail.tracking.unsubcribes' to enable or disable unsubscribes tracking
• Added Enum EventType 'unsubscribes'
• Added Mailgun webhook URL registration using mail:webhooks console command

First release

v1.0.0

Update run-tests.yml