[FE] Instance stats: Issues vs MRs line chart

Problem to solve

In &4147 (closed), we describe the need for an instance-level overview of instance activity.

This is the frontend portion.

backend is tracked in #246846 (closed)

Intended users

Proposal

Screenshot_2020-09-07_at_09.53._2x

  • Add issues vs MRs line chart underneath projects vs groups chart on the instance statistics page
Edited by Martin Wortschack