feat(schedule): add user (un)favorite management

- [x] display/update (un)favorite talk state in schedule page > eg: http://www.devoxx.be/dv13-schedule.html
- [x] display/update (un)favorite talk state in talk page > eg: http://www.devoxx.be/dv13-filip-maelbrancke.html?presId=3667

/!\ cannot test this on staging because eventId is empty now /!\

merging is up to you @stephanj @jayv