Add body blocking

[pulsejet]

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