This commit adds code for live-updating the realm-level default settings page which contains only display settings as of now. This commit also adds realm_user_settings_defaults object to zpage_params so that we can write the tests.
We add a new user_settings module similar to page_params module in frontend and use it to access user's personal settings instead of page_params.
Signed-off-by: Anders Kaseorg <anders@zulip.com>