Add auth_plug to Enable Authentication

Once the `AUTH_API_KEY` PR https://github.com/dwyl/auth/pull/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 https://github.com/dwyl/auth/issues/31
But for now we can simply restrict access based on `conn.assigns.person.email =~ "@dwyl.com"` 👍