Tags may be saved with identical names, case-insensitive

[blms]

testing notes

In the QA admin site, find an existing tag.

• [x] Using the "Add tag" button, try to add a new tag with the same name. You should receive an error on save.
• [x] Try the same name, but use different capitalization from the original tag. You should still receive an error on save.
• [x] Create a tag with a completely different name and save it. It should save successfully. Now, on the edit page for your new tag, rename it to be the same name as a different existing tag, but with different capitalization. You should again receive an error on save.

dev notes

It is currently possible to save two tags with the same name, with the only difference being that different letters are capitalized. Since Taggit uses the query tag = manager.get(name__iexact=name) when TAGGIT_CASE_INSENSITIVE=True, saving a document with one of these tags results in a MultipleObjectsReturned error. Simplest solution is to use case-insensitive uniqueness validation.

example: https://geniza.princeton.edu/admin/taggit/tag/?q=judaeo-arabic+poetry

[kseniaryzhova]

@blms works as expected! Closing, thank you!