Add group-level wiki

Problem

While each git repository has it's own git powered wiki, but there is cases where it would make more sense to have wiki contained in the group that covers several projects. An example link of how this could be accessed done is as follows...

https://gitlab.com/gitlab-org/wikis/home

Work and epics

• MVC 1 - Group-level wiki - Support one wiki at group level: &2214 (closed)
• MVC 2 - Feature parity with project wikis (git, search) - &2782

Proposal from @jareko

• Prototype with artboards and specs: https://sketch.cloud/s/z3oja/a/mxl9jk/play
• Designs for review and discussion