Enhance pluralization support

One of our translations is raising the issue that our current pluralizations only support `one` or `other` and we could be more thorough. The full range is: zero, one, two, few, many, other. (https://www.unicode.org/cldr/charts/47/supplemental/language_plural_rules.html)

We should add them all, and use the Transifex's support for plural resources directly, to simplify things.
