We should allow instance admins to choose a different publishing interface on the web. This builds off our work with the alternative "Classic Editor" on Write.as.
The WriteFreely editor was designed for Write.as, as a simple, distraction-free editor that fit most general use cases. Today WriteFreely is used in many situations where each instance has different needs, for example, for a more spartan or richer writing experience. We should accommodate those uses with the admin ability to choose between editors.
Add a new editor config value to the [app] section that lets admins specify the template file to load.