Add sidebar swapper

[nginx-jack]

Proposed changes

Add sidebar swapper.
Expected behaviour:

• On visiting the page without having used useNewTheme, the current side bar should be visible
• After running useNewTheme(true) in your browser console, the sidebar should be replaced with the text "Sidebar placeholder" and the new theme should be visible
• Running useNewTheme(false) should revert back to the current styling
• Refreshing the page should keep the same state as before the refresh, be it theme true or theme false

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[lamATnginx]

LGTM. Works when trying with Luke to show mainframe