Subscription to issue/mr

Fixes #1911 and #1909

![joxi_screenshot_1426601822159](https://dev.gitlab.org/gitlab/gitlabhq/uploads/53021bc5783271322ab2dfba7598eaa3/joxi_screenshot_1426601822159.png)

![joxi_screenshot_1426601836423](https://dev.gitlab.org/gitlab/gitlabhq/uploads/244ff360fbd6f30980f8dad699400814/joxi_screenshot_1426601836423.png)

See merge request !1702