Fix missing link on public organizations page

When viewing a conference, the Instance Name in the header points to
/organizations. On the organizations index, there's a button to see
conferences, but it does nothing. This makes it easy for public users to
get lost.

This commit:
* Adds a function to the organizations controller to show child
  conferences
* Adds a public permission to see conferences through an organization
* Represents a subset of conferences, reusing conferences/index view,
  for the organization

spec/features/organization_spec.rb

@@ -48,4 +48,12 @@ feature Organization do
 
     it_behaves_like 'successfully updates an organization'
   end
+
+  context 'anonymously' do
+    scenario 'index should link to conferences list' do
+      visit organizations_path
+
+      expect(page).to have_link('Conferences', href: "/organizations/#{organization.id}/conferences")
+    end
+  end
 end