Change to unified settings

[zerolab]

This PR moves the various disparate settings to a single, namespaced settings dict

WAGTAIL_HEADLESS_PREVIEW = {
    "CLIENT_URLS": {},  # defaults to an empty dict. You must at the very least define the default client URL.
    "LIVE_PREVIEW": False,  # set to True to enable live preview functionality
    "SERVE_BASE_URL": None,  # can be used for HeadlessServeMixin
    "REDIRECT_ON_PREVIEW": False,  # set to True to redirect to the preview instead of using the Wagtail default mechanism
}