Answer only to the host names of Bindery

Without a login, a web page that the user visits could reach Bindery
under a name of its own that points at the address of Bindery, and read
and change everything. The "none" mode now answers only to addresses
and to names of a private network. ALLOWED_HOSTS sets the names for
each mode.

tests/test_config.py

@@ -75,7 +75,9 @@ def test_values_from_the_environment(monkeypatch, reload_config):
     monkeypatch.setenv("CALIBRE_DB_PATH", "/library/metadata.db")
     monkeypatch.setenv("BINDERY_USERNAME", " reader ")
     monkeypatch.setenv("BINDERY_PASSWORD_HASH", " hash ")
+    monkeypatch.setenv("ALLOWED_HOSTS", "Bindery.Example.com, .home.example")
     reload_config()
+    assert config.ALLOWED_HOSTS == ["bindery.example.com", ".home.example"]
     assert config.URL_BASE == "/bindery"
     assert config.AUTH_METHOD == "cwa"
     assert config.UPDATE_INTERVAL_HOURS == 1.5
@@ -98,6 +100,7 @@ def test_defaults(monkeypatch, reload_config):
         "LOG_LEVEL",
         "PROXY_AUTH_USER_HEADER",
         "PROXY_AUTH_TRUSTED_IPS",
+        "ALLOWED_HOSTS",
         "SESSION_COOKIE_SECURE",
         "TRUST_PROXY",
         "CWA_DB_PATH",
@@ -123,6 +126,7 @@ def test_defaults(monkeypatch, reload_config):
         "fc00::/7",
         "fe80::/10",
     ]
+    assert config.ALLOWED_HOSTS == []
     assert config.SESSION_COOKIE_SECURE is False
     assert config.TRUST_PROXY is False
     assert config.CWA_DB_PATH is None