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/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