refactor(themes): address published theme assets via PUBLIC_THEMES_PATH
Published assets and language flags are served over HTTP, so they resolve from
PUBLIC_THEMES_PATH. The list of available themes keeps using THEMES_PATH: it is
built from the theme templates, which stay outside the document root.
PUBLIC_THEMES_PATH. The list of available themes keeps using THEMES_PATH: it is
built from the theme templates, which stay outside the document root.