Add notification features for Talk Managers

[jrrickerson]

Create an opt-in notification feature for Talk Managers, so they can be notified of talk-related events.

Proposed design (modifications or suggestions are welcome!):

• Create a Notification Preferences model, which has a link to the User model. The admin page for Notification Preferences should only show the instance attached to the currently logged in user, unless that user is a superuser.
• Provide a list of events the user can turn on or off for notifications. For example:
  • A new talk was submitted
  • A talk was updated / modified
  • A talk was booked
  • A talk was canceled
  • A new session was added
  • A session is coming up which has no booked talks (based on time)
  • A talk is available for an upcoming session but hasn't been booked for any session (based on time)
• Provide a list of notification methods
  • Web notification while logged into admin (https://docs.djangoproject.com/en/1.11/ref/contrib/messages/ for example)
  • Email notifications
  • Text notifications

For implementation, the following would be required:

• For email notification, an email provider, configuration for Django's email system, and a set of email message templates for the various events.
• For text notifications, an SMS provider (like Twilio), configuration, and a set of message templates.
• For time-based notifications (ie. "one week until the next session!") a background worker or cron job will have to be enabled to check times and dates.
• The volume of notifications may warrant having the events pushed onto a queuing system and processed by a background worker.