Add iconic_taxon_name to observations.taxonomy response

[kueda]

This would allow me to use the iconic_taxon_name to provide some flexibility to translators. In Russian and Ukrainian there are different words for "phylum" depending on whether they are plants or animals, so this would allow us to provide iconic_taxon as an inflection for translation keys like ranks.x_phyla that we use on the Lifelists in the Rails app, e.g. @iconic_taxon{Plantae:отдел|Fungi:отдел|Animalia:Тип|Aves:Тип|one Тип / отдел} (they would need to customize the rest of the animals too).

My main unknown is whether adding this join is a performance risk.

[pleary]

I think this is OK to merge. We currently don't have an index on taxa.iconic_taxon_id, but the psql query optimizer seems to be leveraging caching to make the join more performant. We can keep an eye on performance of that endpoint after deploying this, and if we notice slowdowns I'd recommend we index that column. I think the only place we use that endpoint it in production is in lifelists