Dry and test reducers

[effa]

• factor out dispatching actions from reducers
• factor out common code from solve_task_instance and give_up_task_instance
• factor out common code from increase_[started|given_up|solved]_count (maybe just one function with partial application in the stats reducer)
• (maybe even creating/updating entities)

Related question: how to organize/split reducers.py? different pieces: top reducers, delegated functions (by actions, by substate - 2 orthogonal criteria), WORLD_REDUCER (with combine reducers).