* Add polymorphism to Broadcasts table
- Adds a broadcastable_id column to Broadcasts
- Adds a broadcastable_type column to Broadcasts
* Create Announcements and Welcome Notifications tables
- Adds an Announcements table
- Adds a Welcome Notifications table
- Adds timestamps to both tables
- Adds banner_style column to Announcements table
* Adds Announcement and Welcome Notification models
- Adds an Announcement model in prep for polymorphism
- Adds a WelcomeNotification model in prep for polymorphism
* Add data_update script to backfill type_of column for Broadcasts
* Add unique index on broadcastable_type and broadcastable_id on the Broadcasts table
* Add update! to the data_update script to backfill broadcastable_type
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