[wip] Push Subscription Management Proof

[ArnyminerZ]

No description provided.

[rfc2822]

So now that we have a proof 😃 we should do this properly. The first and most important issue I see now is that we when syncable collections change, the PushRegistrationWorker has to be enqueued. My suggestion is:

1. Make sure that all collection DB access (CollectionDao) is done using DavCollectionRepository.
2. Now we have a centralized location that is responsible for collection changes. So we can add observer functionality that allows observer to be notified about changes in collections.
3. Add an observer that listens for changes in syncable collections and enqueues the worker.

What do you think of it? Or do you think there's an easier / better solution?

If we use this solution, I suggest to create a separate issue/PR for the observer and then only do point 3 in this PR.