Fixes #6606 - Allow deletion of non-admin users if 1 admin

When deleting users, you get an error message if there's only one admin account
even if the users aren't admins.
i18n - updated readme instructions
fixes #6375 - fix needed for Rails 3.2.8 only that ensures reference_id on parameter.rb matches the nested object id
fixes #5140 - User children taxonomies did not show tooltip 'Parent is already selected'