Add 'delete' activity for anotations, add an 'oaeId' as a unique annotation identifier

This PR adds the following:

- DELETE push notification when a notification gets deleted. Note that the current implementation aggressively comments out a line that wasn't working for this type of notification and creates an error in the activity feed.
- Persists an `oaeId` property that is a unique ID that the UI posts when the activity is created. This unique ID is used to reference annotations that have been created in order to be able to update their location, highlights,... on the fly.

The following is missing from the PR to be complete:
- Proper fix for the delete push notification
- The `oaeId` probably isn't an ideal solution but the UI will need a similar approach to be able to identify annotations even before they have been added to Cassandra in order to be able to update the UI without the need for a reload + to be able to update other user's documents over push.
- Tests