Unify database configuration files

We had three files that were almost identical, and we can use
environment variables to specify the differences.

Note we're using the `PGUSER` and `PGPASSWORD` variables, since these
variables will automatically be used by the PostgreSQL client when we
have a blank `username` and `password` keys in the `database.yml` file
(which we did until now). The difference between these variables and the
`POSTGRES_USER` and `POSTGRES_PASSWORD` variables is that the `PG`
variables are used by the client connecting to the database, while the
`POSTGRES_` variables are used by the Docker postgresql image when
creating the database superuser.

For consistency with the code in our github workflows (and everywhere
else in the postgres world), we're respecting this double standard. The
fact that there are two different names for what's basically the same
thing makes the code confusing, though, particularly when running the
docker-compose commands, since we get the password from an environment
variable but we have to assign two different environment variables with
it.

So we're accepting both `PGPASSWORD` and `POSTGRES_PASSWORD` variables
in the database configuration file. This way, developers using
docker-compose can use `POSTGRES_PASSWORD` for everything and it'll work
fine. We're also making `PGPASSWORD` default to `POSTGRES_PASSWORD` so
we don't get a warning if we only set `POSTGRES_PASSWORD`:

```
WARN[0000] The "PGPASSWORD" variable is not set. Defaulting to a blank
string.
```

Also note we're using `DB_HOST` instead of `PGHOST` because that's the
variable Rails currently uses by default for new applications [1].

Finally, note we're using `.presence` in the `ENV` calls in the
database.yml file. The `PGPASSWORD` variable was set to an empty string
when running docker-compose, so using `ENV["PGPASSWORD"] ||` wouldn't
work.

[1] https://github.com/rails/rails/blob/c90a8701e5/railties/lib/rails/generators/rails/app/templates/config/databases/postgresql.yml.tt#L22

docker-compose.yml

@@ -6,6 +6,7 @@ services:
     # see https://hub.docker.com/_/postgres/
     image: postgres:13.16
     environment:
+      - POSTGRES_USER=postgres
       - POSTGRES_PASSWORD=$POSTGRES_PASSWORD
     # persist the database between containers by storing it in a volume
     volumes:
@@ -34,6 +35,9 @@ services:
       - bundle:/usr/local/bundle
       - node_modules:/var/www/consul/node_modules
     environment:
+      - DB_HOST=database # Same name as the database service
+      - PGUSER=postgres
+      - PGPASSWORD=${PGPASSWORD-$POSTGRES_PASSWORD}
       - POSTGRES_PASSWORD=$POSTGRES_PASSWORD
 volumes:
   db_data: {}