Stop passing / using object when all we need is the id

Stop passing / using object when all we need is the id

Rather than set id on the contribution object just to be able to access it via contribution->id
let's name the param we keep using & use that. Note I'm still getting contribution->id from contribution
here but I think this makes it clear that the object is mostly only used in addActivity now.

The sligtly larger change is in updateMembershipBasedOnCompletionOfContribution where there is
an instantiation of 'self()' since we no longer have the object