Show PR review messages in the activity feed

Created by: ftsamis

Since GitHub now supports reviews, it'd be nice to show " reviewed #<pr_id>" messages in the activity feed when someone reviews a PR.

Maybe also making the messages more informational such as:

• "user requested changes for #pr"
• "user approved #pr"

Subset of https://github.com/gitterHQ/gitter/issues/633

Created by: ftsamis

Sorry for opening a new issue, a quick search didn't uncover #633 for me.