Removes the customer's logo and returns the path it removed, null when
there was none; throws when S3 refuses the delete, leaving the logo set.
Stores an uploaded PNG, JPEG, WebP or SVG as the customer's logo, a WebP of exactly CUSTOMER_LOGO_TARGET, and an SVG itself beside it; deletes the logo it replaces.
Customer logos on the static-assets CDN (static.cfx.to), under an opaque key per upload, so anything handed the URL (Pylon, an email) can fetch it. The path is stored as the customer's dashboard workspace's
logo_path.