Blueprints: use humanmade/WordPress-Importer in the importFile step

[adamziel]

The importFile step works by interacting with HTML form like a user would. Problem is, the WXR importer requires some interactions with the forms which we fake by adjusting the POST data before submitting the form. This approach causes a number of issues:

• Lack of good error messages since they're all a part of the HTML markup
• wp-now: importFile step fails with error "DOMParser is not defined" playground-tools#210
• Blueprints: Isomorphic DOM - DOMParser, FormData, fetch #379
• Users are not imported on importFile step  #1158
• Random user with name 1 is generated #1161

It seems like the humanmade/WordPress-Importer WXR importer provides a PHP API we could use to configure and run the importing process using just PHP, solving all of the above issues:

$options = array(
	'fetch_attachments' => true,
	'default_author' => $admin_id
);
$importer = new WXR_Importer( $options );
$result = $importer->import( $path );

cc @brandonpayton

[flexseth]

Looks clean!

$options = array(
	'fetch_attachments' => true,
	'default_author' => $admin_id
);
$importer = new WXR_Importer( $options );
$result = $importer->import( $path );