Fix markdown conflicts

Here's the test case I have that duplicates this issue:
1. Install Jetpack and enable markdown
2. Create post in Gutenberg with a couple paragraphs.
3. Open post in classic editor and click update
4. One big paragraph.

This change adds the filter on `wp_insert_post_data` since
edits in classic editor do not go through the rest API.

Additionally, it changes the call to remove markdown which
is more inclusive of all the actions and filters this markdown
implementation touches.

lib/plugin-compat.php

@@ -25,10 +25,12 @@
  * @param  array $post      Post object which contains content to check for block.
  * @return array $post      Post object.
  */
-function gutenberg_remove_wpcom_markdown_support( $post ) {
-	if ( gutenberg_content_has_blocks( $post->post_content ) ) {
-		remove_post_type_support( 'post', 'wpcom-markdown' );
+function gutenberg_remove_wpcom_markdown_support( $post_data ) {
+	if ( gutenberg_content_has_blocks( implode( "\n", $post_data ) ) ) {
+        if ( class_exists ( 'WPCom_Markdown' ) ) {
+            WPCom_Markdown::get_instance()->unload_markdown_for_posts();
+        }
 	}
-	return $post;
+	return $post_data;
 }
-add_filter( 'rest_pre_insert_post', 'gutenberg_remove_wpcom_markdown_support' );
+add_filter( 'wp_insert_post_data', 'gutenberg_remove_wpcom_markdown_support', 1, 1 );