Add tunnel account logic for User model.

This commit adds tunnel account functionality for the User model. This is currently used by the 'Activate/Deactivate' button only but it will be used by the Celery worker through task_resources in the future.

Tunnel credentials are stored encrypted using Fernet as discussed this issue.

Be aware that this commit requires new configuration parameters to be set. These are included in the ldh_middleware sample file for reference: TUNNEL_HOST, TUNNEL_SECRET, TUNNEL_IDENTITY and TUNNEL_KEY.

Next commits will add REST API functionality to get tunnel credentials.