Show the number of mappings in Org and Site #show

I find myself trying to remember if a site has any mappings before clicking the
mappings button.
Move to a LEFT JOIN'd scope for mapping counts

Saves > 100 queries for FCO sites and other large orgs
Merge pull request #145 from alphagov/left-join-mapping-count

Move to a LEFT JOIN'd scope for mapping counts
Merge pull request #140 from alphagov/add_mappings_count_to_org_show

Show the number of mappings in Org and Site #show