Order everything by timestamp rather than ID

[timgl]

The /batch api allows users to give us a specific timestamp. That means the ordering of the events by ID will fall down, as you could insert events out of order.

Places where we order by ID

• action.py
• event.py
• funnel.py
• paths.py
• models.py (filter_by_action and query_db_by_action)
• Possibly some other places

I think this also means we have to create an index on timestamp to make sure this stays fast.