Use eventID rather than the object in completeTransaction

Use eventID rather than the object in completeTransaction

This allows us to stop passing it in as an object as a follow up. It potentially does an extra query but this is
mitigated by
1) it only does a look up if the contribution does not already have a source (rare)
2) it uses a pseudoconstant so would only look up a given event once

- we can later remove the whole loading of event and just get the id from the participant record