Create the admin auth file on deploy

[waldoj]

The .users file in /admin/ is not, of course, stored in Git. But it's not stored anywhere else, either. This requires that it be recreated manually each time the site is deployed to a fresh server. Create this on each deploy.

• store the password as a secret in Travis
• store .users, sans password, in Git
• figure out if a hashed htpasswd password can be moved between systems (or if there's a local salt)
• populate the password in the .users file within Travis