Add interface for creating new realms.

This is controlled by settings.OPEN_REALM_CREATION; if that setting is
off, this feature doesn't do anything.

zproject/settings.py

@@ -142,6 +142,7 @@ DEFAULT_SETTINGS = {'TWITTER_CONSUMER_KEY': '',
                     'ZULIP_COM': False,
                     'ZULIP_COM_STAGING': False,
                     'STATSD_HOST': '',
+                    'OPEN_REALM_CREATION': False,
                     'REMOTE_POSTGRES_HOST': '',
                     'REMOTE_POSTGRES_SSLMODE': '',
                     'GOOGLE_CLIENT_ID': '',