Move the analytics event for submit story to js

See #118
Only log mixpanel submit story event if leader

We have added a leader construct to the players in the game. This will
make it easy to only do an action once with the leader's game.
Merge branch 'feature/mixpanel-change-events' into develop
Merge branch 'develop'

Closes #118
Merge pull request #137 from wlaurance/mixpanel-fix

Mixpanel fix