formatBrandingSettings was updated when the BrandingSettings
interface added the footer-overhaul fields (#441 / #440), so the
admin BrandingPage initialised them as empty strings on every load.
Saving any other field then sent the form's empty socials /
promo_markdown / promo_position back to the backend and wiped the
saved values from the DB. The public gallery footer kept rendering
the old values until the next save, which is why the bug appeared
asymmetric (visible to galleries, gone from the admin form).
Add the missing read mappings for the seven branding_* keys so the
form round-trips them correctly.
Reported by @Rekoo-PS in #460 (split out of #447).
Paul Nothaft
ae64a6acbc