Re-support project issues statistics

[OOnogul]

Project issues statistics was supported by a method (geProjectIssuesStatistics()) for #424
but then with #480 the method was removed either by a mistake or on purpose. (Commit: 203e968)

https://docs.gitlab.com/ee/api/issues_statistics.html#get-project-issues-statistics

Thanks.

[jabby]

I will investigate why this method is not available anymore. As Greg is passed away, it will take some times to find the reason of the removal.

[OOnogul]

I am truly sorry for your loss. I had no idea. Please accept my condolences.

[filipowm]

It seems this can be safely restored, as when I restored it locally, it just worked.