Untranslated taxonomies have inaccurate term post counts

Idea: change "untranslated taxonomies" so the shadow taxonomies are still created. Then we sync the term data within the term translation group (i.e. copy the term name, term slug, term description; in the current taxonomy data structure this means they will all share the same row in `wp_terms`), and we sync the relationships within the post translation group (so all posts in a translation group are associated with the appropriate term in the term translation group).

Related: #75

Review the proposal and related issue #75 first, then trace how untranslated taxonomies, term translation groups, and post translation groups are currently represented. Done means shadow taxonomies remain available, translated term data is synchronized within each term group, and translated posts have the appropriate term relationships.

Written by the indexing model from the issue text.