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/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