Make improvements to the WP.org Openverse site

[dhruvkb]

This PR (​https://github.com/WordPress/wordpress.org/pull/51)

• changes supplied path to lowercase to prevent 404 on case variations of https://wordpress.org/openverse
• uses a fixed value of --header-height to make the iframe occupy 100vh - header height
• adds functionality for sending meta-data from iframe to the parent window
• updates test page (also adds tips for certain fields)
• updates README.md and .wp-env.json to setup a dev environment

This PR

• changes supplied path to lowercase to prevent 404 on case variations of https://wordpress.org/openverse
• uses a fixed value of --header-height to make the iframe occupy 100vh - header height
• adds functionality for sending meta-data from iframe to the parent window
• updates test page (also adds tips for certain fields)
• updates README.md and .wp-env.json to setup a dev environment

Closes https://meta.trac.wordpress.org/ticket/6041

[iandunn]

In 11466:

Openverse: Fix 404s and embed height, send frame meta data

• changes supplied path to lowercase to prevent 404 on case variations of ​https://wordpress.org/openverse
• uses a fixed value of --header-height to make the iframe occupy 100vh - header height
• adds functionality for sending meta-data from iframe to the parent window
• updates test page (also adds tips for certain fields)
• updates README.md and .wp-env.json to setup a dev environment

Props dhruvkb, sarayourfriend, zackkrida
Fixes #6041

Merged in https://meta.trac.wordpress.org/changeset/11466, with Zack's px suggestion included

Thanks @iandunn!