Check for duplicates in taxonomy admin

[actlikewill]

This bug occurs when adding a dupilcate taxonomy from the admin.
Adding validation via clean on the model doesn't work as expected, because treebeard has some extra logic in the save method in MoveNodeForm. it ends up overriding the native behaviour that catches validation errors thrown from the model. Having the logic on the form works better, with some extra work to get the appropriate parent to validate the slug.

closes #1915

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ KeyError: 'name' /admin/peachjam/taxonomy/add/ View Issue

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