Multiple runs of prerender causes flicker

[rbren]

Ran into one issue when running the prerender script: if the homepage is prerendered, and the prerender script is run multiple times, it will cause a flickering issue on all prerendered pages.

More details:

• The normal build process generates dist/browser/index.html
• Prerender script reads dist/browser/index.html and uses that to generate new HTML files
• If homepage is prerendered, dist/browser/index.html is overwritten, changing behavior on subsequent prerender runs

Suggested fix

Have the prerender script make a copy of dist/browser/index.html at dist/browser/original_index.html, and load that file instead of index.html if it exists

[rbren]

PS - I'm happy to implement and send a PR if you guys think this is the right solution.