refs #2174 display site groups and show aggregated values for them

core/Site.php

plugins/MultiSites/MultiSites.php

...s/multisites-dashboard/multisites-dashboard-controller.js

...ts/multisites-dashboard/multisites-dashboard-directive.js

...ripts/multisites-dashboard/multisites-dashboard-filter.js

...cripts/multisites-dashboard/multisites-dashboard-model.js

...avascripts/multisites-dashboard/multisites-dashboard.html

...avascripts/multisites-dashboard/multisites-dashboard.less

.../javascripts/multisites-site/multisites-site-directive.js

...ltiSites/javascripts/multisites-site/multisites-site.html