Datepicker should always return Arabic digits given props.backendDateStandard

Summary: Datepicker may return non-Arabic digits when given a backendDateStandard prop. Not cool, Datepicker. Not. Cool.

Details: props.backendDateStandard implicitly expects the numbering system to always be Latn, i.e. the Arabic numerals 0-9. In fact, in a locale with non-Arabic numerals (such as ar-SA), it still applies the formatter but without converting the numbering system from non-Latn to Latn, so you'd get the right format but with the wrong numerals (think of it as getting Cyrillic letters when you were expecting Latin letters).

Additional reading: Numbering systems in JS, parts of locale strings.

Interested parties: JohnC, ostephens