Try to detect pluralizations when exporting a locale csv file

If a key from the yml file of the source looks like it only contains pluralization keys, instead of exporting those keys we replace them with the pluralization keys from I18n.t('i18n.plural.keys') for the target locale.
Translations export rake tasks have a prereq on :environment

The rails i18n gem adds the pluralization data for the locales, and this doesn't happen if we don't invoke the environment to set things up.  There's also no easy way to do it manually as it's all inside a railtie.
Regen all locales to include proper pluralization forms
Merge pull request #315 from alphagov/export_pluralization_forms

https://www.pivotaltracker.com/story/show/45608311