Slight change to Taxon.single_taxon_for_name.
Lots of small fixes.
* assessments/show format restriction
* prefer showing listed taxon info on obs/show that has introduced / endemic
  status
* pagination bug on taxon_schemes/show when filtering by taxon
* TaxonName.choose_common_name should not choose invalid common names
* filter out "host" from params used for URL generation