Incorrect width attributes in srcset URLs

WordPress prevents images from overflowing the editor by automatically constraining them to $content_width (or 500px). We don’t want that in this case, so we’ll set the constrained size arbitrarily high.

This disables the image size constraint while creating the `srcset`, and fixes the incorrect width attributes.

Note that if/when this merges into WordPress, we'll probably want to handle this in `image_constrain_size_for_editor` (or somewhere), rather than adding in this filter.