Don't create shared terms

[scribu]

Step 1: Split shared terms: #5809

Step 2: Don't create new shared terms: this ticket.

Step 3: Profit.

[SergeyBiryukov]

Related: #22023

[SergeyBiryukov]

#23199 was marked as a duplicate.

[alexvorn2]

this should have a higher priority

[boonebgorges]

The recent patches on #5809 will resolve this ticket.

[boonebgorges]

In 30240:

Do not create shared taxonomy terms.

A "shared" term occurs when two entries in the wp_term_taxonomy table share a
single term_id, and thereby correspond to the same row in wp_terms. This
changeset stops the practice of creating shared terms: each new row in
wp_term_taxonomy will receive its own row in wp_terms. The new strategy
for term creation depends on whether the installation's database schema is up
to date for 4.1:

• If so, terms are allowed to be created with the same slug as an existing term, as long as they are in different taxonomies and do not share a parent. Thus, a new tag with the slug 'wordpress' can exist alongside a category with the slug 'wordpress'.
• If not, new terms will be forced to have unique slugs. Thus, on an installation containing a category with the slug 'wordpress', a new tag 'WordPress' will get the slug 'wordpress-2'.

Fixes #21950. See #5809.