Presenting name for user stream description

This arose from the issue of a user's name is not required to be present and therefore where it's used in the app, it should fallback to the user's username.  I noted this on [meta](http://meta.discourse.org/t/missing-user-value-in-chinese-localized-page/7406/8).

This isn't the cleanest JS because it's my first Ember contribution.  It should probably be moved to the User model and be used throughout the app when a name is required.