Arabic (ar) pluralization incorrect in message file

### Problem

A native Arabic speaker should verify this, but the current declaration of `Plural-forms` appears incorrect for Arabic and it should be six forms, not three, per https://github.com/ergenius/gettext-po-samples/blob/596e24c081dd88cf4644dd4d11ff90de1814daad/po/ar.po#L13 and some other Internet references.

The canonical CLDR definition is here: https://www.unicode.org/cldr/charts/48/supplemental/language_plural_rules.html#ar

The current declaration is: https://github.com/internetarchive/openlibrary/blob/4f4def6691ac3fda9b747f1775613812f5b1d53b/openlibrary/i18n/ar/messages.po#L15

and should instead be:
```
"Plural-Forms: nplurals=6; plural=((n == 0) ? 0 : ((n == 1) ? 1 : ((n == 2) ? 2 : ((n % 100 >= 3 && n % 100 <= 10) ? 3 : ((n % 100 >= 11 && n % 100 <= 99) ? 4 : 5)))));\n"
```

### Reproducing the bug

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