Add body blocking

Create endpoints to allow users to block bodies, wherein they won't see any event for the body anywhere, except when the event is opened directly. Implement this by imposing a large penalty in the prioritizer if the body or any parent is blocked.
* `POST /api/bodies/{uuid}{?block}` where block is `1` or `0`
* When returning the body with any serializer, a `blocked: true` should be present *only if* the body is blocked, i.e. there should be no `blocked: false`

#17