Add issue edited timestamp

Comments already have this feature.
So, I think, issues should also have it.
![Снимок экрана 2014-11-08 в 17.07.10](https://gitlab.com/uploads/vernik91/gitlab-ce/4e87a15130/%D0%A1%D0%BD%D0%B8%D0%BC%D0%BE%D0%BA_%D1%8D%D0%BA%D1%80%D0%B0%D0%BD%D0%B0_2014-11-08_%D0%B2_17.07.10.png)

See merge request !235