Bug 932258 - [WIP] Recurring Calendar Events

### Summary

+ Add markup for repeat selector in `modify_event.html`
+ Change `ModifyEvent#formData` to grab the selected event repeat value
+ Bump db to v16 and add a migration that adds a 'recurrences' field to each event
+ Add a recurrences field to event model
+ Implement `Local#ensureRecurrencesExpanded`
+ Add moment.js for date addition
+ Make event creation trigger busytime expansion

### TODO

+ Busytimes don't show up in UI
+ `Local#expandRecurringEvents` does not get triggered by recurring event creation.
+ Make sure alarms get handled correctly
+ Integration tests for creating local, recurring event
+ Integration tests for creating remote, recurring event