Align analytics event naming with rails client 4.0

[jmileham]

We switched to default snake-case analytics event names, and started sending feature_gate_experienced events in lieu of split_assigned events for feature gates (splits ending in _enabled), because they don't occur just once per visitor anymore, and it could create an impression that it's not working as intended to people looking at the analytics exhaust data from feature gates - even though you shouldn't be attempting to analyze a feature gate as an experiment.