Fixes the following bug when creating a template for a single post:

1. Create post with a long non-latin title: รีวิว Secretlab MAGNUS Pro Lamborghini Edition: เมื่อโต๊ะเกมมิ่งไม่ใช่แค่โต๊ะ แต่คือ “Supercar” ในห้องคุณ
2. Go to Site Editor, Templates, click "Add Template", choose "Single item: Post", for a specific post, and choose the new post with the long title as the post that the template applies to.
3. Creating the template will fail, the REST request will return a 500 internal server error and there will be a PHP fatal on the server.

What happened?

1. The post has a long title, and the database column for post slugs (wp_posts.post_name) is limited to 200 chars. The new post's slug will be longer than that, especially when urlencoded. But the PHP sanitize_title_with_dashes function in Core will correctly truncate it to 200 chars, carefully respecting boundaries of utf-8 multiple-byte characters and of urlencoded octets.
2. Things get worse when Gutenberg tries to create a template from this post, generating a slug with a prefix: single-post-${ postSlug }. Here we are acting on urlencoded postSlug and the urlencoded template slug (now longer than 200 chars) will be sent as payload of the REST request that creates the template.
3. Unfortunately sanitize_title_with_dashes is careful only when dealing with a string that's not urlencoded. It correctly truncates and encodes raw utf-8 characters. But an urlencoded string will be truncated bluntly, and we're left with a slug that's not a valid urlencoded string. It looks something like ...%b8%81%e.
4. Things will start going downhill. The create_item function of the templates controller will crash. Requests to DELETE the new template will be rejected by the Apache server itself, because the URL contains the wrongly encoded slug.

I'm fixing this by decoding the original post slug before appending the prefix. Then the REST request will contain a slug string with raw utf-8 characters, and sanitize_title_with_dashes will be able to truncate it nicely.

We should also do a companion Core patch that addresses the crashes. But this Gutenberg fix is good enough to get rid of the issue.