Disallow creation of robots.txt in freeurl

Add a configuration setting to "hard"-disable creation of notes as
set by the configuration value. This defaults to `['robots.txt',
'favicon.ico']`, because these files are often accidentally created
by bots and browsers.

This commit fixes #1052.

Signed-off-by: Daan Sprenkels <hello@dsprenkels.com>

lib/response.js

@@ -157,7 +157,7 @@ function findNote (req, res, callback, include) {
       include: include || null
     }).then(function (note) {
       if (!note) {
-        if (config.allowFreeURL && noteId) {
+        if (config.allowFreeURL && noteId && !config.forbiddenNoteIDs.includes(noteId)) {
           req.alias = noteId
           return newNote(req, res)
         } else {