Add a tag_versions table for tracking the history of tags.
A couple notable differences from other version tables:
* There is a previous_version_id column that points to the previous
version. This allows finding the first, last, previous, or next
version efficiently in SQL.
* There is a `version` column that tracks the revision number (1, 2, 3, etc).
Post versions and note versions have this, but other version tables don't.
* The `updater_id` column is optional. This is because we don't know who
the last updater was before we started tracking the history of tags,
so the initial updater will be NULL in the first version of the tag.
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