Add ssh key attribute to return attributes

I realized that https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/712 would cause a regression with the SSH public key sync. This merge request fixes that functionality. 

Also, this uses the `Module#prepend` method that @DouweM told me about a while ago. It ends up being a **really** slick way to override a CE method and make future merges really smooth. I like this a lot!

Part of this has to be changed in CE because the `#user_attributes` method didn't exist before. I made that change here, and in CE at <LINK MR HERE>.

@DouweM What do you think about this usage of `prepend`?

cc/ @jacobvosmaer-gitlab

See merge request !736