The menu UI selector is not scalable

Problem/Motivation

🐛 When adding a new menu link, restrict the available parents to the current menu Fixed fixed this for the new custom menu link use case. This bug still exists when editing menu links.

Proposed resolution

Provide a more scalable UX for moving menu links from menu to another. There should be one field which is used for selecting the menu, and one field for selecting the actual parent element.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet