Notify email to mentioned users; show participants ala github on issuables

This implements http://feedback.gitlab.com/forums/176466-general/suggestions/3726309-email-notify-user-when-they-are-mentioned-anywher

Also adds rendering of issuables participants ala github. 

![participants](https://f.cloud.github.com/assets/517228/491211/ca2ec1b2-b9f9-11e2-84dd-e8e8ad15b533.jpg)