Fneves/failing deploys summary

This pull request adds an alert icon next to each deployment reference on summary page to each deployment where the last deployment was not successful.
When hovered, a popover will show when, by who and what release failed to deploy.
When clicked samson will navigate to the failed deployment page.

Screenshot:

![failed_deployment](https://cloud.githubusercontent.com/assets/647255/5244140/0a2f1cd4-7944-11e4-9836-d84d64d83aa1.png)


/cc @zendesk/samson @pswadi-zendesk 

### References
 - Jira link: [SAMSON-124](https://zendesk.atlassian.net/browse/SAMSON-124) 

### Risks
 - None

`stage.last_deploy.try(:failed?)`