Migration script to handle existing custom taxonomies

[dkoo]

#39 deprecates the custom category and tag taxonomies for listings only, in favor of post categories and tags. This PR will be a breaking change, so we should consider creating a migration script to help sites who may be using those taxonomies. This script should get all listing posts that have terms from the deprecated taxonomies and then:

1. Check for a corresponding post category or tag term with the same name, and if it exists, assign that
2. If a corresponding post category or tag term doesn't exist with the same name, create and assign it
3. (optionally) remove the term assignments for the deprecated taxonomies from any posts that have them

Tagged as on hold so we can confirm whether this is needed first.

[matticbot]

🎉 This issue has been resolved in version 2.0.0-alpha.1 🎉

The release is available on GitHub release

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