Added review and mark test data generators

Change-Id: Idfb43d1e86854a4f2a4ab868aeaec57ce9fd97bd
Fixed handler of engineer contribution stats

Split stats retrieving on /stats/engineer (used by charts and tables)
and /stats/engineers_extended (used in contribution report)
Added tests for these handlers.

Change-Id: Ia839ffeac52984c2d63690ef5bc473450c0d46ee