Refuse OPDS logins when CWA accepts every password

A CWA with anonymous browsing can answer an OPDS request whatever the
password, so each password logged a user in to Bindery. Bindery now
tests CWA with a login that cannot exist, and refuses OPDS logins if
CWA accepts it. A login check no longer shares the session of the
account of Bindery. The CWA account stays optional for a CWA that asks
for no login, and a missing account now gets its own message.

tests/test_calibre_db.py

@@ -53,6 +53,7 @@ def db_login(client, user_db, monkeypatch):
     monkeypatch.setattr(config, "CWA_DB_PATH", user_db)
     remote = Mock(spec=CwaClient)
     remote.login_ok.return_value = True
+    remote.accepts_any_login.return_value = False
     monkeypatch.setattr(orchestrator, "cwa", lambda: remote)
     return remote