REST API: Terms: Respect taxonomy's default query args

[ockham]

Port of WordPress/gutenberg#67154.

What

It is possible to supply a set of default query args to register_taxonomy() which will be used when querying a list of terms -- for example, orderby in order to specify how the resulting list of terms should be sorted. This PR makes it so that the list of terms returned by the Terms REST API controller respects that order if no post ID is included in the REST API request.

How

The controller's get_items() method has two code branches:

1. If a post ID is given in the network request, get_items() uses wp_get_object_terms() to get the terms associated with the given post. That function already includes code to respect the default query args specified when the taxonomy was registered.
2. If no post ID is given, it uses get_terms(), which does not respect the default query args.

This PR adds code to respect the default query args if no post ID is given.

Trac ticket: https://core.trac.wordpress.org/ticket/62500

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[ockham]

I'll go ahead and merge this per WordPress/gutenberg#67154 (review):

[T]he Core backport in #7848 also makes perfect sense. The REST controller should apply the taxonomy's default args.

[ockham]

Committed to Core in https://core.trac.wordpress.org/changeset/59458.