R1:3ab6a7e19f46

Generate "stub" events earlier, so more infrastructure works with Calendar

Summary:
Ref T9275. When you create a recurring event which recurs forever, we want to avoid writing an infinite number of rows to the database.

Currently, we write a row to the database right before you edit the event. Until then, we refer to it as `E123/999` or whatever ("instance 999 of event 123").

This creates a big mess with trying to make recurring events work with EditEngine, Subscriptions, Projects, Flags, Tokens, etc…