Scope issue projects to a Group when possible

When using IssuableFinder with a Group we can greatly reduce the amount
of projects operated on (due to not including all public/internal
projects) by simply passing it down to the ProjectsFinder class.

This reduces the timings of the involved queries from roughly 300
ms to roughly 20 ms.

Fixes gitlab-org/gitlab-ce#4071, gitlab-org/gitlab-ce#3707