Group project contributions by both name and email.

See https://twitter.com/Argorain/status/598751646143897600.

We now group by both name and email, so that "Douwe Maan - douwe@gitlab.com", "Douwe Maan - me@douwe.me" and "Douwe M. - me@douwe.me" are all combined into one graph.

Fixes internal issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2328.

See merge request !700