Add/allow import site gutenberg pr

[smithjw1]

Motivation for the change, related issues

This is related to supporting visual regression testing of Gutenberg PRs with Playground.
WordPress/gutenberg#62729

Importing content into a Playground makes testing easier. This will allow the supported import-site parameter to be used on this page as well.

Implementation details

This change adds a check for a the import-site query parameter and adds a step to the blueprint when it is found.

The link has minimal validation, but I don't believe this creates a significant security concern. In theory someone could corrupt the blueprint by directing people to this URL with a malformed query parameter.

Validating that the parameter is a URL, and not passing the query string or hash seemed to mitigate the most obvious offenses.

Testing Instructions (or ideally a Blueprint)

1. Run locally
2. Visit gutenberg.html with a query parameter import-site pointing to a valid Playground ZIP import.
3. Confirm that Playground has imported content from ZIP file.

[bgrgicak]

Thank you @smithjw1, the PR looks good!

[adamziel]

@smithjw1 what's the use-case for this vs including that step in the provided Blueprint before redirecting to gutenberg.html?

[smithjw1]

I'm not sure I understand the question. My understanding of this page is that it builds a blueprint, encodes it into a URL, and redirects you to a Playground with that blueprint.

I didn't see a way in the code to get just the blueprint itself as something to be manipulated.

[adamziel]

@smithjw1 aha, I got confused here. Thank you for explaining!