Add label notes

It creates system note, when someone changed labels of issue/MR.

Just look at the screenshot:

![labels](https://gitlab.com/uploads/vernik91/gitlab-ce/ebbdba6bdc/labels.png)

Solves this [request] (http://feedback.gitlab.com/forums/176466-general/suggestions/6623258-create-a-note-when-issue-labels-change).

*P.S. Thanks this [PR](https://github.com/gitlabhq/gitlabhq/pull/8237) for idea 😉. But I need this feature as soon as possible.*

See merge request !331