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/sponsors/_form.html.haml

@@ -0,0 +1,19 @@
+.row
+  .col-md-12
+    .page-header
+      -if @sponsor.new_record?
+        New
+      %h1
+        Sponsor
+        = @sponsor.name
+.row
+  .col-md-8
+    = semantic_form_for(@sponsor, :url => (@sponsor.new_record? ? admin_conference_sponsors_path : admin_conference_sponsor_path(@conference.short_title, @sponsor))) do |f|
+      = f.input :name
+      = f.input :description
+      = image_tag f.object.logo(:thumb) if !f.object.logo.blank?
+      = f.input :logo
+      = f.input :website_url
+      = f.input :sponsorship_level, collection: @conference.sponsorship_levels
+      %p.text-right
+        = f.action :submit, as: :button, button_html: { class: 'btn btn-primary' }