REPLACES the story panel's brand block (logo, title, subtitle). The host owns the whole
block, so LoginBannerTitle / LoginBannerSubtitle / LoginBannerLogoLabel no longer apply.
ADDITIVE, after the brand block. Suppresses the 'banner'-region MJLoginCards, which
are the stock content for this region — a host that wants both renders the cards' equivalent
in its own template.
ADDITIVE, above the sign-in options. Empty by default.
ADDITIVE, below the sign-in options. Suppresses the 'panel'-region MJLoginCards,
on the same rule as 'bannerFooter'.
The regions of the login screen a host can fill with its own markup.
Two of the four replace what the screen would otherwise draw, and two are additive — the distinction is stated per slot below rather than left to be discovered, because it decides whether a host's content lands beside the stock content or instead of it.