Show user roles by comments.

I think it would be useful to show the user's role (Owner/Master/Developer/Reporter) by comments on GitLab.com (like GitHub does with Owner/Collaborator). That way people will know that when someone from the project's core team comments, they're not just some random user, but they're actually on the team. Currently they have the gather this from context.

![Screen_Shot_2015-05-12_at_12.15.45](https://gitlab.com/gitlab-org/gitlab-ce/uploads/0da0eea2087d6d3b14a2c0c594575c8a/Screen_Shot_2015-05-12_at_12.15.45.png)

On hover, the note actions replace the role:

![Screen_Shot_2015-05-12_at_12.17.32](https://gitlab.com/gitlab-org/gitlab-ce/uploads/54a83c2f5047ea3ce0387f6692b14fe5/Screen_Shot_2015-05-12_at_12.17.32.png)

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Addresses internal issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2321.

cc @sytses

See merge request !639