Support Users Directly Uploading Profile Pictures.

* Adds a `picture` attribute to `User`.
* Preserves Gravatar as a fallback option.
* Now, you should use `user.profile_picturer` instead of `gravatar_url`
* TODO: Ensure `profile_picture` handles sizing right for uploaded images.

app/views/admin/conferences/_top_submitter.html.haml

@@ -6,7 +6,7 @@
       - @top_submitter.each do |key, value|
         .row.top-submitter
           .col-md-2
-            = image_tag(key.gravatar_url(size: '25'), title: "Yo #{key.name}!", alt: '', 'class' => 'img-circle img-responsive text-center')
+            = image_tag(key.profile_picture(size: '25'), title: "Yo #{key.name}!", alt: '', 'class' => 'img-circle img-responsive text-center')
           .col-md-10
             %h4
               = link_to key.name, admin_user_path(key)