Add API endpoint for obtaining build's ActT

[magnusbaeck]

As of #21 it'll be possible for clients to obtain the ActT event of a build via the APIs by digging up the EiffelActivityTriggerAction and its event attribute, but that's not a great API. We should provide a proper REST endpoint that exposes this information. That'll e.g. make it easier to change how the event is serialized (see review comment).