Improve menu parent link selection

Problem/Motivation

So while creating/editing the menu link, the Parent item select input of the Menu Settings opens up a very long list of the every single possible menu item in one giant select list which can be a little crazy.

Steps to reproduce

- Navigate to edit any menu link.
- Open the Parent link select list.

Proposed resolution

So the proposed solution that we came up with after having a discussion with @lauriii was to have 2 separate select list. The first select list will contain the List of main menus and based on the selection of the first select list the second select list would contain it’s subitems.

Remaining tasks

1. Accessibility review
2. If #2519362: Redesign the menu link add form to be less overwhelming is fixed before this issue, then make sure the two work well together. See Comment #37.

User interface changes

Current

Proposed

API changes

Data model changes