A repository's own look for its shared demo pages

A repository can now give the demo pages it shares its own look with a single file, .obeya/theme.css, on its default branch. The theme sets colours, fonts and a logo. Files it names inside .obeya/ are embedded in the page; anything else is dropped, even references written in roundabout ways, because the theme is read the way a browser reads CSS. Pages of repositories without a theme stay exactly as before. When the theme changes, pages already shared show as outdated and can be shared again.