Configurable BASE_URL setting

The BASE_URL setting is so far used when sending notifications, you can use it to customise the notification body using the {base_url} token.

The BASE_URL is set initially in your environment vars

export BASE_URL=https://mysite.com:5000

or from your docker-compose.yml recipe.

The BASE_URL setting can also be overridden in the Settings page.

Don't forget to include the :port number