Plural messages without a translated value did not correctly fallback to
the message key as singular messages do. This caused undesirable
behavior when working with plural messages.

I've shifted the plural form resolution into the Translator. Now that we
have a custom translator class this logic is better suited here instead
of having duplicate logic in each message formatter. Interestingly our
formatters had divergent logic for handling plurals which could cause
some tedious to fix issues for app developers.

I've not unset the `_singular` key in the Translator as userland formatters could be
relying on those keys being passed in.

Refs #10900