Add to SILENCED_SYSTEM_CHECKS from environment

Allow the environment to silence more system checks, used by dockerflow
for __heartbeat__. Ensure "models.W040" is included, even if not
specified.

privaterelay/settings.py

@@ -456,11 +456,14 @@ def _get_initial_middleware() -> list[str]:
 if TEST_DB_NAME:
     DATABASES["default"]["TEST"] = {"NAME": TEST_DB_NAME}
 
-SILENCED_SYSTEM_CHECKS = [
-    # (models.W040) SQLite does not support indexes with non-key columns.
-    # RelayAddress index idx_ra_created_by_addon uses this for PostgreSQL.
-    "models.W040",
-]
+SILENCED_SYSTEM_CHECKS = sorted(
+    set(config("DJANGO_SILENCED_SYSTEM_CHECKS", default="", cast=Csv()))
+    | {
+        # (models.W040) SQLite does not support indexes with non-key columns.
+        # RelayAddress index idx_ra_created_by_addon uses this for PostgreSQL.
+        "models.W040",
+    }
+)
 
 REDIS_URL = config("REDIS_URL", "")
 if REDIS_URL: