Improve Taxon validations and factory

[RyanofWoods]

Summary

Fixes #4849

To resolve the two main issues outlined in #4849. This will help prevent stores from creating problematic Taxons when creating them manually as well as making it easier to setup Taxons correctly via its factory.

Checklist

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Release notes

New Taxon and Taxonomy validations

New validations were added to the Taxon and Taxonomy models but are behind the new temporary extra_taxon_validations and extra_taxonomy_validations preferences.

• Taxonomy, must have a unique name;
• Taxon, must have a unique name under the same parent Taxon (or at root level); and
• Taxon, a validation was added to ensure Taxonomies can only have 1 root,

Due to the upgrading process, you have the opportunity to introduce these preferences and validations at a later time after upgrading to 3.4.

Before using the new preferences via uncommenting them in new_solidus_defaults or loading 3.4 defaults, ensure you don't have any now invalid Taxons or Taxonomies on production.

You can do this by:

1. Pulling the production Taxons and Taxonomies to a development or staging environment;
2. Setting the preferences to true;
3. Running Spree::Taxon.all.select(&:valid?) and Spree::Taxonomy.all.select(&:valid?); and
4. If you don't have empty arrays, you will need to manually fix these records on production (warning you may have products attached to these Taxons/Taxonomies).

Notes:

• If you had any invalid records, take caution in the case that you have any code that creates Taxons or Taxonomies.
• This may break some of your tests if you had incorrectly built Taxons, see Taxon factory changes or easy ways to resolve this.

Improved Taxon testing factory

• Now, if you pass just a parent Taxon, the Taxonomy will be inferred from the parent;
• As before, if you pass no taxonomy, a Taxonomy and its root Taxon will be made;
• Now, the parent will be inferred from the given taxonomy or default created Taxonomy - this means, the created Taxon will always be nested (If you want a root Taxon, create a Taxonomy and get its root);

These changes help to create valid Taxons and makes setup easier.

[RyanofWoods]

It was difficult to understand the use of the spec that was deleted in Remove unuseful Taxon spec. Discussion about this spec would be appreciated 🙏

[kennyadsl]

Love it! I've been hit by this issue in the past, thanks for taking care of this. I left some comments but overall this is a great change I look forward to merge.

[kennyadsl]

@RyanofWoods would love to move this forward. Can you please rebase?

[RyanofWoods]

I rebased and added one new commit @kennyadsl. It fixes a mistake I made in the Taxon factory. On approval, I can squash it into the Improve Taxon factory commit.