Expose combined diffStat on Campaign(Plan)

Created by: eseliger

Exposing a combined diffStat on the Campaign and CampaignPlan resources would help not having to fetch all changesets just to show the diff indicator for the campaign. This is really redundant as we move to using the FilteredConnection component for diffs on campaigns, that is able to lazy load more content. Hence the diff would not be the correct number until all entries have been loaded. This is a step to mitigate performance issues when we have larger diffs