Cannot embed example sketches

[catarak]

Nature of issue?

• Found a bug

Details about the bug:

• Web browser and version: Chrome
• Operating System: Mac OSX
• Steps to reproduce this bug:

1. Open an example, such as https://editor.p5js.org/p5/sketches/Hello_P5:_song
2. Change the URL to https://editor.p5js.org/p5/embed/Hello_P5:_song
3. You will see a 404 error