AJAX Thumbnail Rebuild

High risk: 1 removed function, 1 removed class, 1 removed AJAX action.

• The function ajax_thumbnail_rebuild_ajax() was removed. Direct calls from a theme or another plugin will fatal.
• The class AjaxThumbnailRebuild was removed. Code that instantiates or extends it will fatal.
• The AJAX action wp_ajax_ajax_thumbnail_rebuild was removed.
• This release introduces exec() (shell execution), which the previous version did not use. Review before deploying.
• New external hostnames appear in this release: api.shortpixel.com, api.tinify.com, wsrv.nl.
• outbound requests — This plugin did not make outbound HTTP requests before. It does now.