Upgrade to 1.0.0-rc864

Concerns: pushword/advanced-main-image

The "∅" main image format is now labelled None

The format that hides the main image shows as None (Aucun in French) in the admin instead of the ∅ symbol, and flat files export mainImageFormat: None. Files still holding mainImageFormat: ∅ keep importing as that format, so there is nothing to run; the next pw:flat:sync export only rewrites those lines. A site that overrode adminPageMainImageFormatNone in its own translations keeps its own label.