AO3-7135 Record canonized/decanonized dates on tags

[pmonfort]

Pull Request Checklist

• Have you read "How to write the perfect pull request"?
• Have you read the contributing guidelines?
• Have you added tests for any changed functionality?
• Have you added the Jira issue number as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
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Issue

https://otwarchive.atlassian.net/browse/AO3-7135

Purpose

Adds two new datetime columns (canonized_at and decanonized_at) to the tags table to record when a tag was most recently canonized or decanonized.

• A before_save callback sets canonized_at when a tag becomes canonical, and decanonized_at when it becomes non-canonical.
• The tag edit page displays "Last canonized on [datetime] UTC" and "Last decanonized on [datetime] UTC" to admins (superadmin, policy_and_abuse, tag_wrangling), directly below the "last edited" information. Sentences are omitted when the corresponding date is nil.
• Both fields are indexed in Elasticsearch and exposed as sort options ("Date Canonized", "Date Decanonized") in tag search.

Testing Instructions

1. Log in as a tag wrangler.
2. Create a new tag (non-canonical). Edit it, you should not see "Last canonized on" or "Last decanonized on".
3. Create a new canonical tag. Edit it, you should not see the dates (wranglers don't see them).
4. Log out.
5. Log in as an admin with superadmin, policy_and_abuse, or tag_wrangling role.
6. Edit the non-canonical tag from step 2, you should not see any canonization dates.
7. Check "Canonical", save, and re-edit, you should see "Last canonized on [datetime] UTC"
8. Uncheck "Canonical", save, and re-edit, you should see both "Last canonized on" and "Las
9. Edit the canonical tag from step 3, you should see "Last canonized on [datetime] UTC" on
10. Find a tag that was canonized before this deployment and decanonize it, it should shotetime] UTC" only (no canonized date since that happened before the migration).
11. Log out.
12. Search > Tags, verify "Date Canonized" and "Date Decanonized" appear in the "Sort by" d
13. Sort by "Date Canonized" descending, recently canonized tags should appear first.
14. Sort by "Date Decanonized" descending, recently decanonized tags should appear first.

Credit

Pablo Monfort (he/him)