Create completely blank page to develop and test front-end code in isolation

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As part of the acceptance criteria for Rapid Diffs we need a way to isolate the application from the rest of the GitLab product.

Criteria 3 and 4 both set very low limits for bytes to load, which would be hard to tune in the noise of a typical page load (approximately 3MB on production).
Criteria 6 requires this isolation as a matter of fact.

Some guidelines:

• When the page finishes loading, the browser network tab should look something like this:
  
  • Note that the favicon load is initiated automatically by the browser.
• We must be able to add arbitrary CSS and JavaScript to this page
  • This must skip a compilation step (e.g. Webpack/Vite), which would add a huge amount of weight and bloat to the compiled file
    • There needs to be a location (e.g. some kind of /public folder) where we can drop test files that can be loaded directly by the native browser module loader.
• We must be able to opt back in to some add-ons arbitrarily.
  • For example:
    • we may want to test with adding the standard GitLab authentication to the page
    • we may want to test re-adding the global stylesheet (e.g. gitlab-ui to the page)
    • etc.