Add cospeaker support to reviews feedback system

[lukeman]

Not receiving notifications that the primary speaker received. E.g. an email went out about a meeting that all speakers were supposed to attend, but co-speakers didn't get it.

Cospeakers should also be able to respond to questions from reviewers.

[miurahr]

This may realize it https://github.com/pinax/symposion/pull/81

[ossanna16]

Can this issue be closed? Seems like it's been fixed by @miurahr 😊