Introduce a 'publicly_queryable' argument to register_taxonomy

[Chouby]

Since #21949, it is no more possible to query non-public taxonomies.

Instead of controlling whether a taxonomy can be queried directly with the public argument, I propose to handle this the same way it is already done for post types.

A new argument publicly_queryable controls whether it is possible to query the taxonomy on front end. The public argument controls the default value for publicly_queryable, show_ui, and show_in_nav_menus.

[boonebgorges]

Thanks for the patch, Chouby. It would be nice to have a few tests demonstrating how publicly_queryable gets its default value from public.

[Chouby]

These are my first unit tests so I am not sure that they are pertinent enough.

[Chouby]

attachment:34491.3.patch​ is just a refresh of attachment:34491.2.patch​

[boonebgorges]

In 36525:

Introduce publicly_queryable taxonomy argument.

Taxonomies registered as publicly_queryable can be queried as taxonomy
archives.

If not provided explicitly, the value of publicly_queryable is inherited
from public.

Props Chouby.
Fixes #34491.

[DrewAPicture]

I do believe we need a changelog entry for this new argument :-)

[boonebgorges]

Whoops. Usually I remember.

[boonebgorges]

In 36564:

Add changelog entry for publicly_queryable argument in register_taxonomy().

Missed in [36525].

Fixes #34491.