Add CWA_ENABLED to run Bindery without CWA

With CWA_ENABLED=false, Bindery ignores the CWA settings, makes no request
to CWA, and shows nothing about CWA in the log and the web interface. A
set INGEST_DIR still gets each changed book as a plain epub.

Without it, the log names each CWA function that a missing setting turns
off.

tests/test_api_flows.py

@@ -46,6 +46,7 @@ def test_add_list_download_and_cancel_jobs(client, story):
     assert result.json["mode"] == "metadata"
     app_config = client.get("/api/config").json
     assert app_config["metadata_fields"]["title"]
+    assert app_config["cwa_enabled"] is True
     assert (app_config["story_jobs_hours"], app_config["story_jobs_count"]) == (36, 10)
     assert (app_config["main_jobs_hours"], app_config["main_jobs_count"]) == (6, 25)
     assert (app_config["job_list_max_hours"], app_config["job_list_max_count"]) == (720, 500)