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/controllers/organizations_controller.rb

@@ -4,4 +4,10 @@ class OrganizationsController < ApplicationController
   def index
     @organizations = Organization.all
   end
+
+  def conferences
+    @current = @organization.conferences.upcoming.reorder(start_date: :asc)
+    @antiquated = @organization.conferences.past
+    render '/conferences/index'
+  end
 end

app/models/ability.rb

@@ -15,7 +15,7 @@ class Ability
 
   # Abilities for not signed in users (guests)
   def not_signed_in
-    can [:index], Organization
+    can [:index, :conferences], Organization
     can [:index], Conference
     can [:show], Conference do |conference|
       conference.splashpage && conference.splashpage.public == true

app/views/conferences/index.html.haml

@@ -4,7 +4,7 @@
       .page-header
         %h2 Upcoming Conferences
   - @current.each do |conference|
-    = render partial: 'conference_details', locals: { conference: conference }
+    = render '/conferences/conference_details', conference: conference
   -if @antiquated and @antiquated.any?
     .row
       .col-md-12
@@ -17,7 +17,7 @@
             %i.fa.fa-chevron-down{ style: 'display: none' }
     #antiquated.collapse
       - @antiquated.each do |conference|
-        = render partial: 'conference_details', locals: { conference: conference}
+        = render '/conferences/conference_details', conference: conference
 
 -content_for :script_body do
   :javascript

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'