Send notifications of upcoming or new events to subscribers

[pauljacobson]

The idea is to create a mechanism to send notifications to subscribers of upcoming and/or new events. We need the following:

1. All future events

• Current date
• Date of future event

2. New events since last notification

• Current date
• Date of future event
• Add "Date Added" field to all events

3. All future calls for papers

• Current date
• Date of future calls for papers

4. New calls for papers since last notification

• Current date
• Date of future calls for papers
• Add "Date Added" field to all calls for papers