Fix embedded not london.gov.uk site issues and remove docs

[PaulioRandall]

What does this change and why?

1. Fixes an issue where an application embedded in a non-london.gov.uk website wouldn't load cookie consent properly.
2. Moved general doc comment to Svelte script as comments in head scripts are not stripped by Svelte compiler.
3. Optimised code to be more concise and readable.

How is it tested?

Within live Voter ID app.

How is it documented?

Storybook and code comments.

Is it complete?

• [x] Have you included changeset file?