Add auth_plug to Enable Authentication

[nelsonic]

Once the AUTH_API_KEY PR dwyl/auth#43 is merged,

• Add auth_plug to enable Auth in the Email app.
• Display Decrypt Email address to an Authenticated person with @dwyl.com email address.

We could add more sophisticated RBAC once it's available dwyl/auth#31
But for now we can simply restrict access based on conn.assigns.person.email =~ "@dwyl.com" 👍