Only show organisations with sites

* We’ve got hundreds of organisations now, if they don’t have any sites
being transitioned they shouldn’t be shown
* When the list grows long again we’ll need a better solution, probably
a filter
Add a test for orgs without sites
Org controller spec needs sites to pass now
Add an Organisation.with_sites scope
Fix the headline message

* Was displaying a hash after transition to with_sites scope
Remove unbalanced closing tag
Stop issuing N org queries per org#index
Merge pull request #108 from alphagov/hide-orgs-without-sites

Only show organisations with sites