Provide upgrade profile that reverts the hiding of the discussion viewlet, fixes #12.

Note that this may un-hide other belowcontent viewlets as well. YMMV.

This does not provide a full 0.3.1 -> 2.0 upgrade, but at least it clears the way to
get a working installation again after manual reconfiguration of @@disqus-controlpanel.

To reproduce #12 and the fix:
- bin/buildout -c old-0.3.1.cfg
- add site, install disqus, enable discussion, configure disqus
- add NewsItem, enable discussion -> you have a disqus form
- bin/buildout to enable 2.0
- configure disqus, view NewsItem -> no form
- ZMI portal_setup import disqus upgrade
- view NewsItem -> now it works