Taxonomy: Redirect to current taxonomy when adding a term without AJAX.

Fixes a regression in which adding terms via a full page refresh, ie without AJAX, would always redirect to the tags taxonomy.

Props szaqal21.
Fixes #39328 for trunk.

Built from https://develop.svn.wordpress.org/trunk@39649


git-svn-id: https://core.svn.wordpress.org/trunk@39589 1a063a9b-81f0-0310-95a4-ce76da25c4cd