Problematic Taxons can be created

[RyanofWoods]

A lot of these problems are caused by incorrectly creating Taxons via code rather than using the admin page, but some simple validations could prevent these issues.

Solidus Version:
all/master

Issues

1. A Taxonomy should have one root Taxon, but still multiple Taxon's can be created with a parent_id of nil.
   

   solidus/core/app/models/spree/taxonomy.rb

   Line 10 in 2b79f72

   has_one :root, -> { where parent_id: nil }, class_name: "Spree::Taxon", dependent: :destroy

2. You can make "duplicate" Taxons, i.e. Taxons that have the same name and parent.
   These both can be fixed with simple validation.

3. If you have taxons with the same lft and rgt attributes, deleting one, deletes all of them. I am not really sure how this occurred on the project I am working on, but just wanted to share, while I figure out the cause.

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I am working on a PR for the first and second issues