Multi object recurrences

@wesleykendall This pull request adds support for tracking multiple 'objects' with a single recurrence. It does this by adding an additional model, `RecurrenceForObject`, which includes a foreign key to the recurrence, and a generic foreign key for whatever objects you want associated with that recurrence.

The intention is to make group subscriptions to triggers a lot easier. Because we didn't have group subscriptions worked out yet, we ended up with some hacks in triggers to keep things running, which introduced dependencies on ambition.