[5.x] Fix CP nav ordering for when preferences are stored in JSON SQL columns

[jesseleite]

At a high level, this PR changes the YAML schema in saved CP Nav preferences to use a new array format...

nav:
  reorder:
    - fields
    - content
  sections:
    ...

Reordering the old way required object key order to be preserved, but this breaks down once saved to JSON column in MySQL, and it tries to order keys alphabetically instead...

nav:
  reorder: true
  sections:
    fields: @inherit
    content: @inherit # This would end up first, because `c` comes before `f`

This new format solves that ordering issue in SQL, but should also work just fine in flat file scenarios. I have also taken care to ensure the old legacy format can still be read when building navs from existing users' preferences as well.

TODO:

• Save new schema to YAML
  • At the section level
  • At the item level
  • At the deeper child level
• Read new schema from YAML when building nav
  • At the section level
  • At the item level
  • At the deeper child level
• Ensure it can still read the legacy schema when building nav
  • At the section level
  • At the item level
  • At the deeper child level
• Tests