[Core API] Conversation Reminders

Introduced PATCH /conversations/conversation_id/reminders to expose the ability to snooze and unsnooze a conversation. Please see Update conversation reminders ] for more details.

Introduced scheduled_reminders to the conversation model. Please see Conversations for more details.

Introduced reminder as a possible source for archive and reminder type events. Please see Events for more details. The _meta for this source type will match the model described for scheduled_reminder in Conversations.