Add environments_folder_app

What does this MR do and why?

This MR implements #431906 (closed) and is a first step towards refactoring the Environments / Folder page. It adds the empty page component that would host the new version of the page, a feature flag that will be used for the feature rollout, and some feature and unit test spec files.

Screenshots or screen recordings

N/A

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Navigate to a project that has environments with folders. If you don't have one, use the following gitlab-ci configuration:

deploy:dev/static-files-1:
  stage: deploy
  script: echo "done"
  environment: dev/static-files-1

deploy:dev/static-files-2:
  stage: deploy
  script: echo "done"
  environment: dev/static-files-2

deploy:stg/static-files-1:
  stage: deploy
  script: echo "done"
  environment: stg/static-files-1
  
deploy:stg/static-files-2:
  stage: deploy
  script: echo "done"
  environment: stg/static-files-2

deploy:prd/static-files-1:
  stage: deploy
  script: echo "done"
  environment: prd/static-files-1

deploy:prd/static-files-2:
  stage: deploy
  script: echo "done"
  environment: prd/static-files-2

2. Navigate to Environments page
3. Expand an environment folder and click Show All link
4. Check the legacy folders page is still opened
5. Enable a feature toggle:

Feature.enabled?(:environmens_folder_new_look)

6. Check that the new folder page is rendered

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.