Contract: emit activityKey in NewCommitment event

[spengrah]

Currently, the ActivityUpdated and NewCommitment events have the below signatures. Most importantly, NewCommitment only emits the activityName param and not the activityKey param, which forces the subgraph to use activityName as the ID for the Activity entity. Since the contract logic ensures there are no collisions between activityNames this is technically ok, but its more conventional to use something other than plaintext as an entity id.

In the next version of the contract, NewCommitment should emit the activityKey param. For readability on block explorers, it can also emit the plaintext activityName param, though that does carry a small gas cost.

event ActivityUpdated(string name, bytes32 activityKey, address oracle, bool allowed, bool exists);

event NewCommitment(address committer, string activityName, uint256 goalValue, uint256 startTime, uint256 endTime, uint256 stake);