Add observer on org to add/remove corp info from search

It observes updates on organisations and adds corp info pages to the index if the org is changing from govuk_status ??? to 'live'.  It will also remove them from the index if the org is changing from govuk_status 'live' to ???.  Create & destroy events on the corp info page should be handled by the default searchable index_after/unindex_after so we don't need to do them here.
Merge pull request #340 from alphagov/index-corp-info-pages-when-org-goes-live

Add & Remove corp info pages from search when their org changes state

https://www.pivotaltracker.com/story/show/45957935