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.

client/components/users/userHeader.jade

@@ -7,7 +7,7 @@ template(name="headerUserBar")
       else
         = currentUser.username
     a.header-user-bar-avatar.js-change-avatar
-      +userAvatar(user=currentUser)
+      +userAvatar(userId=currentUser._id)
 
 template(name="memberMenuPopup")
   ul.pop-over-list
@@ -33,8 +33,6 @@ template(name="editProfilePopup")
       input.js-profile-initials(type="text" value=profile.initials)
     input.primary.wide(type="submit" value="{{_ 'save'}}")
 
-template(name="changeAvatarPopup")
-
 template(name="changePasswordPopup")
   +atForm(state='changePwd')