Bug 1042939 - Alarms, periodic sync, notifications refactor r=millermedeiros

_Note_: This is a work in progress!

### Summary of changes

There are a few different pieces of functionality affected.

#### Periodic sync

1. When the calendar starts, if periodic sync is enabled, we should write an alarm to mozAlarms to wake us up for periodic sync.
2. If the user changes the periodic sync interval, we should revoke any previous periodic sync alarms and issue a new one at the modified interval.
3. When we get woken up by a periodic sync alarm, sync and then issue a new alarm.

#### Notifications

1. When we receive an event reminder from the alarms api, send a notification via the notifications api about the reminder. When that notification gets clicked, navigate to the alarm url.

### What's coming!!

1. Fix for calendar alarm spam.
2.  Don't start periodic sync until we have a calendar that needs a real sync operation.
3. Calendar opens and stays open after using an alarm.
instead of doing the flag check we could have 2 variables for the sync frequency (`prevSyncFrequency` and `curSyncFrequency`) while `curSyncFrequency` always stores the latest value, and `prevSyncFrequency` stores the last value used on `scheduleSync`.

that way we can do:

```js
if (prevSyncFrequency !== curSyncFrequency) {
  scheduleSync();
}
```

flags that hold state are a big red flag.. specially when we have it scattered through multiple functions.. in this case we are setting and using the `scheduling` flag in multiple places, so it's very easy to introduce bugs.