As a quick fix, set disqus_identifier to undefined in all cases

This was requested by @evdb while on holiday, to fix the
display of comments.  Apparently contrary to Disqus's
documentation, the path will be disregarded if
disqus_identifier is set.  There's some relevant discussion
here:

  https://github.com/mysociety/mzalendo/issues/421
Merge branch 'master' into production