Implement consistent admin interface

* Consistent route/model/controller/view layout
* Get rid of unused photos, speakers, dietchoices, social_events controllers
* Drop unused :public from Rooms and :description from CallForPapers

app/views/admin/difficulty_levels/_form.html.haml

@@ -0,0 +1,16 @@
+.row
+  .col-md-12
+    .page-header
+      %h1
+        - if @difficulty_level.new_record?
+          New
+        Difficulty Level
+        = @difficulty_level.title
+.row
+  .col-md-8
+    = semantic_form_for(@difficulty_level, :url => (@difficulty_level.new_record? ? admin_conference_difficulty_levels_path : admin_conference_difficulty_level_path(@conference.short_title, @difficulty_level))) do |f|
+      = f.input :title, :required => true
+      = f.input :description, :input_html => {:rows => 3, :class => "span6"}
+      = f.input :color, :input_html => {:size => 6, :type => "color"}
+      %p.text-right
+        = f.action :submit, as: :button, button_html: { class: 'btn btn-primary' }