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/controllers/organizations_controller_spec.rb

@@ -2,6 +2,26 @@ require 'spec_helper'
 
 describe OrganizationsController do
   let!(:organization) { create(:organization) }
+  let!(:conference) do
+    create(
+      :conference,
+      splashpage:   create(:splashpage, public: true),
+      venue:        create(:venue),
+      organization: organization
+    )
+  end
+  let!(:antiquated_conference) do
+    create(
+      :conference,
+      splashpage:   create(:splashpage, public: true),
+      venue:        create(:venue),
+      organization: organization,
+      start_date:   2.weeks.ago,
+      end_date:     1.week.ago
+    )
+  end
+
+  let!(:other_conference) { create(:conference) }
   let!(:user) { create(:user) }
 
   describe 'GET #index' do
@@ -12,4 +32,27 @@ describe OrganizationsController do
 
     it { expect(response).to render_template('index') }
   end
+
+  describe 'GET #conferences' do
+    before :each do
+      get :conferences, id: organization.id
+    end
+
+    it 'loads the organization' do
+      expect(assigns(:organization)).to eq organization
+    end
+
+    it 'includes organization conferences' do
+      expect(assigns(:current)).to include conference
+    end
+
+    it 'does not include conferences outside organization' do
+      expect(assigns(:current)).not_to include other_conference
+      expect(assigns(:antiquated)).not_to include other_conference
+    end
+
+    it 'includes antiquated organization conferences' do
+      expect(assigns(:antiquated)).to include antiquated_conference
+    end
+  end
 end