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.

docker-compose.yml

@@ -5,6 +5,8 @@ services:
     # The user and group that own ./config and can write CWA's ingest folder. Usually the same as CWA's.
     user: "1000:1000"
     environment:
+      # Without CWA: set this, and remove the CWA values below. INGEST_DIR then gets the built ePubs.
+      # CWA_ENABLED: "false"
       # Omit this and its volume mount to keep uploads paused.
       INGEST_DIR: /cwa-book-ingest
       # The address of CWA, as the Bindery container reaches it, and the CWA account that Bindery uses.