[Push] Show notification on push notification (until sync is started)

[rfc2822]

When a push notification comes in, we enqueue a sync worker for the respective account. However, this worker will often not be executed immediately, depending on the priority of the push message, battery saving settings etc.

So we should probably do what Android apps are expected to do [1, 2, 3, 4] when a push notification comes in:

1. Enqueue a one-time sync worker.
2. Show a low-priority notification with some text like "Server-side changes pending".
3. If the user opens DAVx⁵ over the notification, the work will usually start because DAVx⁵ is in foreground. Alternatively, the worker may be as a background job at some time without user interaction.
4. As soon as the worker starts, it should remove the notification.

[rfc2822]

• Channel: CHANNEL_STATUS
• Use setGroup

[rfc2822]

How should the architecture be? Given that:

• push receiver shows "push received – sync pending" notification (per account/authority)
• (local changes may show a "local changes – sync pending" notification, again per account/authority)
• all notifications for a given account/authority should be dismissed when the respective sync is run

[rfc2822]

As we have talked at the conf. Doesn't have to be a perfect design at the first try, but let's try