Press This: some images don't get "sideloaded"

[azaozz]

Seems caused by slashes in post_content. We are looking at the POST which is slashed, need to strip slashes, run WP_Press_This::side_load_images(), then add slashes again.

[azaozz]

In 31799:

Press This:

• Strip slashes while running side_load_images(), add slashes after.
• Simplify and clean up side_load_images().
• Add another arg to media_sideload_image() to return the uploaded image src only, and fix it to always return WP_Error on errors.

Fixes #31660.