Add support for plurals in translations

@frosty In #6208 you introduced the construct:

    if (count == 1) {
        self.noResultsView.titleText = NSLocalizedString(@"You have 1 hidden WordPress site.", "Message informing the user that all of their sites are currently hidden (singular)");
        self.noResultsView.messageText = NSLocalizedString(@"To manage it here, set it to visible.", @"Prompt asking user to make sites visible in order to use them in the app (singular)");
    } else {
        self.noResultsView.titleText = [NSString stringWithFormat:NSLocalizedString(@"You have %lu hidden WordPress sites.", "Message informing the user that all of their sites are currently hidden (plural)"), count];
        self.noResultsView.messageText = NSLocalizedString(@"To manage them here, set them to visible.", @"Prompt asking user to make sites visible in order to use them in the app (plural)");
    }

Unfortunately this cannot be translated properly because some languages don't just have a different plural for count != 1 (for example Russian, they use one form for 1, 21, 31; another one for 2, 3, 4; and yet another one for 5, 6, …). Please see Handling Noun Plurals and Units of Measurement in the iOS docs on how this can be done universally. Thanks!