This PR adds a db table for CR schedules. The table has two columns:
1. `change_request` :: This acts as both a foreign key and as the
primary key for this table.
2. `scheduled_at` :: When the change is scheduled to be applied.
We could use a separate ID column for these rows and put a `unique`
constraint on the `change_request` FK, but I don't think that adds any
more value. However, I'm happy to hear other thoughts around it.
