The setup for refactoring the contextual panel

What does this MR do?

Sets the basis for refactoring the left-hand panel behind a feature flag. What this MR does:

CSS Changes:

• Creates a duplicate of contextual_sidebar.scss as contextual_sidebar_variant.scss
• Re-names original contextual_sidebar.scss into contextual_sidebar_base.scss
• Introduces the style hub to get both stylesheets' styles prefixed based on .sidebar-refactoring applied to the root element of the content part depending on the :sidebar_refactor feature flag

⚠ To test locally, please, enable the :sidebar_refactor feature flag for your local user. The flag is scoped to the user and is not global

Screenshots (strongly suggested)

master	This MR

⚠ Keep in mind that the color changes in the panel are purely for the clear indication of the flag kicking in and will be replaced in the proper refactoring work.

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because the work is behind the feature flag.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

References

• Related to #322680 (closed)
• Follow-up !62254 (merged)