Fix spacing of Clojure code

This fixes the Clojure formatting issue I raised. Feels like it should have been easier! It:

  * Changes Loofah to use a HTML rather than an XML scrubber
  * Temporarily changes Gemfile to use a fork of Rouge that emits valid HTML
  * Makes some minor CSS changes to accomodate the newly valid HTML

Once / if the pull request to Rouge is accepted the Gemfile should be
reverted to use the official version. Happy to do this myself when it happens.