Viewer registration API endpoint

[ZhangYiJiang]

Agenda viewers need to be able to create an 'account' so that they can persist their customizations across sessions and receive updates when their sessions get updated by the organizers

[ZhangYiJiang]

Partially implemented - needs documentation, but here's a summary

• POST /{agenda_id}/viewer - takes in email and returns a token. This token is used to uniquely identify the viewer.
• GET /{agenda_id}/viewer/{token} - returns an object with email and sessions, which is a list of session ID the viewer picked
• PUT/DELETE /{agenda_id}/viewer/{token}/{session_id} - adds or removes a session from a viewer's list of sessions. Returns 204 on success, otherwise 404 if either the agenda_id, token or session_id is invalid.