Compose REDIS_URI from component parts.

This updates the configuration handling for the Redis connection string.
The motivation is so that the Redis password can be managed separately
via Azure Key Vault and eventually be rotated independently of the rest
of the connection URI.

This also tweaks the method we use to build the DATABASE_URI and removes
some stale config from the CI config file.

.circleci/config.yml

@@ -30,7 +30,7 @@ commands:
         default: atat_test
       container_env:
         type: string
-        default: -e PGHOST=postgres -e REDIS_URI=redis://redis:6379
+        default: -e PGHOST=postgres -e REDIS_HOST=redis:6379
     steps:
       - run:
           name: Set up temporary docker network
@@ -172,7 +172,7 @@ jobs:
           command: |
             docker run \
               -e PGHOST=postgres \
-              -e REDIS_URI=redis://redis:6379 \
+              -e REDIS_HOST=redis:6379 \
               --network atat \
               atat:builder \
               /bin/sh -c "pipenv install --dev && /bin/sh script/cibuild"