Make PreviewShape Iframe use post request

[SawyerHood]

Right now we set the src of the iframe to generate the site. I want to be able to not have to url encode all of the other html pages that I send up for the prompt. It would be better if when the form is submit we send a post request and make the target of the output the iframe.

This involves:

• Changing PreviewShape.tsx
• Updating api/html/route.ts to be a POST route