Add option to enable freely permission in closed instance

Before, closed disallowed guest edits completely, by removing
the `freely` permission. This makes it possible to explicitely bring
back guest-editing, but not guest-note-creation, to closed instances.

Signed-off-by: Dario Ernst <dario@kanojo.de>

lib/config/default.js

@@ -16,6 +16,7 @@ module.exports = {
   protocolusessl: false,
   usecdn: true,
   allowanonymous: true,
+  allowanonymousedits: false,
   allowfreeurl: false,
   defaultpermission: 'editable',
   dburl: '',