Fix production environment to use proper secrets

[amcewen]

We'll have a few things: Postgres password; admin username and password; etc. which at present are hard-coded. They should get picked up from the environment, and that should be defined properly in the production environment.

[helicalbytes]

In the absence of a full username / password database (or some other authentication storage) we need to add the following 2 environment variables for the admin pages:

OPTIMISM_ADMIN_USERNAME
OPTIMISM_ADMIN_PASSWORD

The password in particular needs to be secure and be provided from a secret.

[helicalbytes]

When we merge develop to master this will fail.