[PRMP-1372] add pdfjs setup script - add pdfjs build to react app

[mark-start-nhs]

add pdfjs setup script + custom_css to apply,
add iframe of pdfjs to react app,
add pdfjs to nginx.conf + remove some of the deprecated request permission-policies
update tests to point to the new iframe , <-- there needs to be a new test to check the iframe contents
update .gitignore to keep the pdfjs build but not the setup_pdfjs.py temp files,
add gulp-cli as a dev dependency used to compile pdfjs

[AndyFlintNHS]

Overall, it’s a really solid chunk of work I’m happy to go ahead with a few considerations:

1. That we’re happy that the print/rotate buttons for the PDF are hidden in a menu
2. That permitting 'unsafe-inline' is okay within the nginx.conf Content-Security-Policy
3. I worry about maintainability, this approach means that we will have full customisability of our PDF viewer, at the cost of having to run Mark’s setup_pdfjs.py script to update versions and add the entire pdf.js code into our codebase.