Display group README in overview

Frontend implementation for the "Group overview" section of #378737 (closed)

Designs

Group overview

• If a sub-project gitlab-profile exists and contains a README.md, display the README below projects table
• The README header should link to the associated project/README.md

Implementation plan

Started in !110943 (closed)

1. Add Vue app mount element to app/views/groups/show.html.haml
2. Pass group.group_readme&.present.web_path and group.group_readme&.present.name to frontend
3. Use ~/repository/components/preview/index.vue to render the README

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