[MR Analytics] Implement at Group level

Problem to solve

Currently the MR Analytics page (including the Throughput feature) only exists at the Project level. There is considerable interest in offering at the Group level as well. This would make it more useful to customers, as well as to GitLab itself – we measure Throughput of the gitlab-org group as our main productivity metric (not just the gitlab project).

Proposal

Introduce a Group-level MR Analytics page that looks and behaves just like the Project-level page, and is accessible from the Group-level Analytics menu in the left-nav.

Examples

• @ebrinkman wrote: "I'd like to see us start to shift to group level everything."