Improve group overview UI

Problem to solve

With https://gitlab.com/gitlab-org/gitlab-ce/issues/44704 we implement the first step to improve the project overview landing page. On group overview pages, we are currently re-using a very similar design approach. Let's explore which changes we can and should derive for our Groups overview page as well.

Further details

Proposal

We should align the Group overview page with the new Project overview page. However, there is an opportunity to also identify a Group page as such on first sight.

Details to add

What does success look like, and how can we measure that?

(If no way to measure success, link to an issue that will implement a way to measure this)

Links / references

Edited by Andreas Kämmerle