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/js/frontend.test.mjs

@@ -46,7 +46,7 @@ const text = (node) => node instanceof Element ? node.children.map(text).join(''
 const find = (root, tag, label) => elements(root).find((el) => el.tagName === tag && text(el) === label);
 const settle = () => new Promise((resolve) => setImmediate(resolve));
 const config = {
-  metadata_fields: { title: 'Title' }, mode_labels: {}, library_available: true, update_interval: 0,
+  metadata_fields: { title: 'Title' }, mode_labels: {}, library_available: true, cwa_enabled: true, update_interval: 0,
   job_list_max_hours: 720, job_list_max_count: 500,
 };
 const story = {