Ability to disable confidential issues

This affects our use of gitlab.com, thus why I'm ignoring all the setup info.

Recently GitLab added support for confidential issues. These are sometimes not useful for open-source projects, to the point where people opening confidential issues is a problem. Reasons why they may be not wanted:

• The project may want to be transparent
• Confidential issues don't play nice with non-confidential ones (hard to close issues as duplicate, etc)
• Some users tick the box by mistake

I argued in https://gitlab.com/gitlab-org/gitlab-ce/issues/3678#note_16807709 that, since this feature was added, there should be a button to turn it off on a per-repo basis. I would prefer if the option was off by default on new repos, but that's secondary.

Marking as a bug instead of a feature since to me this is a bug - a new feature that we don't want forced itself into our repos and we have no way to remove it.