Re-use GA cookie when sending server-side events to Segment.io

@mulby, could you give this a look?
How would you feel about this?

```python
# These fields are present elsewhere in the event at this point
context_fields_to_remove = set(CONTEXT_FIELDS_TO_INCLUDE)
# These fields are only used by segment.io tracking and do not concern researchers
context_fields_to_remove.add('client_id')
for field in context_fields_to_remove:
```