What does this MR do and why?

Project overview page re-oganisation Part I: Add new sidebar. This MR is part of Project overview page updates (#429186 - closed) in a series of changes related to each other.

Note: This MR moves content into the new right sidebar. It does not alter or remove any content. All changes are behind a new FF project_overview_reorg

Changes:

• Adds new right sidebar
• Adds new project settings button to sidebar
• Moves project description to sidebar
• Moves languages bar to sidebar
• Moves project stats to sidebar
• Moves project buttons to sidebar

Note that this is the first Part and we will iterate on it with upcoming MRs. Also for that reason it does not yet add test coverage for the new layout.

Screenshots or screen recordings

Before	After

Empty project

Before	After

How to set up and validate locally

1. Enable FF echo "Feature.enable(:project_overview_reorg)" | rails c
2. Check out a project overview page, e.g. http://gdk.test:3000/gitlab-org/gitlab-shell
3. Check out an empty project page (or create one)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to Create sidebar to display project overview meta... (#431537 - closed), Project overview page updates (#429186 - closed), ✂️ UX Paper Cuts 16.7 → Project page (#430244 - closed)