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

app/finders/issuable_finder.rb

@@ -81,7 +81,8 @@ class IssuableFinder
     elsif current_user && params[:authorized_only].presence && !current_user_related?
       @projects = current_user.authorized_projects.reorder(nil)
     else
-      @projects = ProjectsFinder.new.execute(current_user).reorder(nil)
+      @projects = ProjectsFinder.new.execute(current_user, group: group).
+        reorder(nil)
     end
   end