Improve performance of getting issues on group level

For testing I used the URL http://localhost:3000/groups/gitlab-org/issues?milestone_title=8.1. Prior to these changes said URL would take about 10-12 seconds to load. By applying these changes the loading time has been reduced to roughly 2-3 seconds. 

There's still some stuff going on in some views that I have to look at, resolving those changes might reduce the loading time a bit more. I also still have to check if I didn't break too many tests.

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

See merge request !2318