Refactor the scheduled items feature

[jguadagno]

This feature is not working as initially intended and needs some rework. Currently, at the scheduled time it takes an item, either a Table or SourceData, and raises an event that the publishers can subscribe to. The publisher then provides the 'content'.

On a new item added...

• Create a series of events: "I'm speaking", "In 90 days", "In 30 days", "Next week", "Tomorrow", "Coming up". This should be with a content publisher
• Each of the content items creates a scheduled_item record.
• When a scheduled item fires, we send then publish the composed message.

These event series could be a 'collection' of messages to send. The events/messages will be created based upon the event type.

[jguadagno]

Currently, when a scheduled_item is found in the database table of ScheduledItems that is due to be sent, a message is sent to the Event Grid topic of scheduled-item-fired which, currently, has three subscribers, Twitter, Facebook, and LinkedIn. The ProcessScheduledItemFired function for each of these publishers will craft a post/status to send to their respective platforms.

We should create or handle the (Process)NewSourceData event for Engagements and Talks and generate the requested scheduled events like "I'm speaking", "In 90 days", etc. Right now, these are manually created records in the database table.