Update Email Settings

Request body for the company's email branding.

The whole configuration, not one named field: the settings page sends the state it wants, so a new setting is a field here rather than an endpoint.

The company's email branding, as the settings page renders it.

Unlike the document-settings response this one can be empty — a company that has never configured branding has no reply-to to invent, and the page needs to know that so it presents the form as unsaved rather than as holding a value nobody chose. configured says which state this is.