Add notification features for Speakers

[jrrickerson]

Related to #23.

For speakers, because they don't have admin access, a view would need to be provided or an area on an existing Profile edit view provided to manage their notification preferences.
Proposed speaker notification events:

• A submitted talk was approved / booked for a session.
• A submitted talk was rejected
• The session for a booked talk is approaching (time based)

For simplicity and privacy Speaker notifications should probably only be delivered via email.