Menus: Add a Cancel link to the screen for creating a new menu.

The use case this helps the most is when there's only one menu and you click "create a new menu". In this case the Cancel link allows you to go back to the single existing menu.

Props garrett-eclipse, audrasjb.
Fixes #46367.
Built from https://develop.svn.wordpress.org/trunk@47770


git-svn-id: https://core.svn.wordpress.org/trunk@47546 1a063a9b-81f0-0310-95a4-ce76da25c4cd