Add a unique key when embedding shortcode

[zackkatz]

Enhance security by requiring a key for embedding the download link shortcode:

• Generate a key in the feed to be used with the shortcode
• Do not require the key for existing feeds
• Require a key for new feeds

See https://gravitykit.slack.com/archives/G59JBVCT0/p1706031560973249

[doekenorg]

This was released in 2.2.0