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.

atst/app.py

@@ -223,20 +223,24 @@ def make_config(direct_config=None):
         config.read_dict({"default": direct_config})
 
     # Assemble DATABASE_URI value
-    database_uri = (
-        "postgres://"
-        + config.get("default", "PGUSER")
-        + ":"
-        + config.get("default", "PGPASSWORD")
-        + "@"
-        + config.get("default", "PGHOST")
-        + ":"
-        + config.get("default", "PGPORT")
-        + "/"
-        + config.get("default", "PGDATABASE")
+    database_uri = "postgres://{}:{}@{}:{}/{}".format(  # pragma: allowlist secret
+        config.get("default", "PGUSER"),
+        config.get("default", "PGPASSWORD"),
+        config.get("default", "PGHOST"),
+        config.get("default", "PGPORT"),
+        config.get("default", "PGDATABASE"),
     )
     config.set("default", "DATABASE_URI", database_uri)
 
+    # Assemble REDIS_URI value
+    redis_uri = "redis{}://{}:{}@{}".format(  # pragma: allowlist secret
+        ("s" if config["default"].getboolean("REDIS_TLS") else ""),
+        (config.get("default", "REDIS_USER") or ""),
+        (config.get("default", "REDIS_PASSWORD") or ""),
+        config.get("default", "REDIS_HOST"),
+    )
+    config.set("default", "REDIS_URI", redis_uri)
+
     return map_config(config)