Add the ability to rename a tag

[simonw]

Add the ability to rename a tag and create a PreviousTagName object recording the previous name.

• Add PreviousTagName model in blog/models.py.
• Add rename_tag method to Tag model in blog/models.py to handle renaming and create PreviousTagName objects.
• Update TagAdmin in blog/admin.py to handle renaming and create PreviousTagName objects.
• Add migration for PreviousTagName model in blog/migrations/0021_previous_tag_name.py.
• Add tests for renaming a tag and creating PreviousTagName object in blog/tests.py.
• Add tests to ensure tags can contain hyphens in blog/tests.py.

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[simonw]

Renamed a ton of tags: https://simonwillison.net/admin/blog/previoustagname/

[simonw]

Here are my changes so far: https://github.com/simonw/simonwillisonblog-backup/commit/d78d6598f5ef6d864be918dea090b03b57c46cfe#diff-3be0327e375535593bb1e958b3bee14521c40f951298d5132f119bf3976d51f8