Feature/disable password login cont

WP [#7626](https://www.openproject.org/work_packages/7626)
For a user having an external authentication provider, this shows a warning:

![screen shot 2014-07-28 at 14 51 22](https://cloud.githubusercontent.com/assets/188986/3720540/29337676-1656-11e4-96ce-4ab6ef8c8631.png)