Added 'external_hashpath.js' for hashnav support

See in-code comments for details. If a custom root page is being used
(instead of the built-in RapidApp login page) the following should be
included in that docuemnt's <head> section to enable external hashnav
naviagtion through '/auth/login'

  <!-- enable root hashnav redirects -->
  <script
    type="text/javascript"
    src="/assets/rapidapp/misc/current/js/external_hashpath.js"
  ></script>

TODO: May consider loading this in a more automated way, but this would
need to be done carefully, if at all (since we want to avoid munging
*custom* pages where we don't have access to the <head> section, etc)