Make private / public repository status more visible (BB-13095)

To avoid confusion over having a public vs. private repository, I think we should change the interface slightly:

Right now we display a lock next to a "private" repository and no lock for a public repository. This isn't always as clear as it should be, so I propose a couple possible changes:

• Use lozenges stating "public" or "private" next to the repository on the dashboard
• Per https://bitbucket.org/site/master/issue/2323/create-a-way-to-group-repositories – Maybe create a separate group / listing for private and public repositories

From a security standpoint, we should also only allow admins to make a repository public (instead of repository creator).