feat(iframe): create iframe shortcode to enforce lazy loading of iframes, fixes #3154

[MichaelSolati]

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[robdodson]

This is looking good but I think we should also add a Glitch shortcode (maybe it can use the IFrame shortcode in its implementation?)

{% Glitch 'fav-kitties' %}

It'll need to support three arguments:

• id (the id of the glitch to display)
• path (this is the file the glitch will display first)
• previewSize (this controls how much of the app preview is visible)

If path and previewSize aren't given then it should omit path from the query string, and set the previewSize to 100