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/realtime.js

@@ -781,7 +781,7 @@ function connection (socket) {
       var note = notes[noteId]
       // Only owner can change permission
       if (note.owner && note.owner === socket.request.user.id) {
-        if (permission === 'freely' && !config.allowanonymous) return
+        if (permission === 'freely' && !config.allowanonymous && !config.allowanonymousedits) return
         note.permission = permission
         models.Note.update({
           permission: permission