FEAT: Automate announcement emails

[TobyHFerguson]

Announcement emails should be:

• sent with a no-reply email address
• automatically scheduled to send 3 days before the event
• created when a ride is scheduled
• updated when a ride is updated
• deleted when a ride is unscheduled
• allow the scheduler to add or edit a portion of the message
• edit the message independently of any updates to the ride (e.g. 'Debbie is bringing cake'!)