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
Sorting by "id" has the same effect as sorting by created_at while
performing far better and without the need of an extra index (in case
one wanted to speed up sorting by "created_at").
Sorting by "Recently updated" still uses the physical "updated_at"
column as there's no way to use the "id" column for this instead.
