Add Hierarchy Page

What does this MR do and why?

Adds Hierarchy to the Group and Project information, and displays existing planning hierarchy. Add a Current structure and Unavailable structure section to this new Hierarchy page.

Issue: #340844 (closed)

What's not done in this MR:

1. Survey banner on hierarchy page
2. Banner on Issues list page

Screenshots or screen recordings

navigation

full page

tiers

free tier	premium tier	ultimate tier

How to set up and validate locally

1. Enable the feature flag Feature.enable :work_items_hierarchy
2. Navigate to a project or group information > hierarchy to view the page.

Switching between licenses

1. Open gitlab repo in a text editor like VS Code. Eg. run code . in the terminal.
2. Hit CMD + P and type in app/assets/javascripts/work_items_hierarchy/work_items_hierarchy_bundle.js. Hit enter to open the file.
3. On line 15 you'll see a line like licensePlan, change it to:
   • licensePlan: 'free', for testing free plan
   • licensePlan: 'premium', for testing as premium plan
   • licensePlan: 'ultimate', for testing as ultimate plan.

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.