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

app/views/organizations/index.html.haml

@@ -12,5 +12,7 @@
           .caption
             %h4
               = organization.name
-            %button.btn.btn-success Conferences
+            = link_to 'Conferences',
+              conferences_organization_path(organization),
+              class: 'btn btn-success'
             / = link_to 'Edit', edit_organization_path(organization), class: 'btn btn-mini btn-default'