Split shared taxonomy terms during term update.

When updating an existing taxonomy term that shares its `term_id` with
another term, we generate a new row in `wp_terms` and associate the updated
term_taxonomy_id with the new term. This separates the terms, such that
updating the name of one term does not change the name of any others.

Note that this term splitting only occurs on installations whose database
schemas have been upgraded to version 30133 or higher. Note also that shared
terms are only split when run through `wp_update_term()`, as on edit-tags.php;
we will wait until a future release of WordPress to force the splitting of all
shared taxonomy terms.

Props boonebgorges, rmccue, greuben, garyc40, wonderboymusic, imath, jesin.
Fixes #5809.
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