Compress images upload via JSON API

[tijmenbruggeman]

Description
This merge request will add an action to rest_api_init to create a compressor. This will ensure that images uploaded via the media endpoints (https://developer.wordpress.org/rest-api/reference/media/) are properly compressed if possible.

Background
Whenever an image is uploaded it will generate various sizes and call the action wp_generate_attachment_metadata (link). This action was called on uploading an image through the REST API and the compress function on Tiny_Image was called, though because no compressor existed in the settings the images were not compressed:

// class-tiny-image.php:197
public function compress() {
	if ( $this->settings->get_compressor() === null || ! $this->file_type_allowed() ) {
		return;
	}
        ...
}

Usually, when admin or ajax is initiated it will call the admin_init or ajax_init functions. These have methods hooked up which will create a compressor. When the request goes to the REST API, nothing was done to create the compressor. The solution was to add an action to the rest_api_init hook, which is called when receiving a new REST request, and make sure the compressor is created.