Hangout on projects

PT story https://www.pivotaltracker.com/story/show/74032336

No changes to UI.
0. You can start hangout from a project's page on wso.  It will post a notification to slack.
1. Event now has_many hangouts
2. All hangouts are now persisted in db with a unique uid and do not get overwritten when restarting a hangout for a new occurrence of an event.
3. Title, category and hangout_url are now updated when HangoutApplication reports back to wso.

Demo at my [my instance](http://yaro.herokuapp.com)

p.s. the first 3 commits are extracted in PR 475.
don't know how you managed it on the HO app, but wouldn't it make much more sense to have the fields `id` and `type` instead of `eventId` and `hangoutId`?