Activity log

Activitiy log feature. These PR are dependent from this PR on comfortable mexican sofa: https://github.com/moneyadviceservice/comfortable-mexican-sofa/pull/11
But this two matchers have the same implementation: https://github.com/rspec/rspec-expectations/blob/master/lib/rspec/matchers/built_in/contain_exactly.rb#L7
Is the reason because is more idiomatic to use the contain_exactly?