Re-factor the avatar system and support avatar uploads

The user is now able to upload an avatar, and pick one in a list.

This functionality should eventually be abstracted in a community
package but we still need to work on a great public API. We rely on
collectionFS to manage uploaded avatars. We also removed
bengott:avatar which was trying to solve the wrong problem (namely
displaying the avatar, which is as simple as displaying an image), and
not a avatar system as it should be.

Gravatar support is coming (back) soon. We may also want to have a
list of default fun avatars the user can choose instead of uploading
its own one.

server/publications/avatars.js

@@ -0,0 +1,3 @@
+Meteor.publish('my-avatars', function() {
+  return Avatars.find({ userId: this.userId });
+});