Improve embed script highlighting and add copy button

[adriaanvanrossum]

• Fix highlighting in dark mode in script embed
• Add copy embed code button

When highlighting a code area in darkmode, it's basically impossible to tell what you've highlighted (in this image the script is highlighted and ready to be copied).

Additionally it would be nice to have a button to copy that script instead of having to highlight it + copy it myself (you do this in other places such as your contact page's company details)

Overall though I absolutely love the product and the website! As a fellow software developer you guys are doing it right.