Fix regression in repeated appendTab()s when navbarMenu() is present

[cpsievert]

Closes #3516

[cpsievert]

This reverts the logic back to the way it was in the 1.6.0 release (

shiny/srcjs/shinyapp.js

Lines 876 to 885 in b57cb6c

	$tabset.find("> li").each(function() {
	var $tab = $(this).find("> a[data-toggle='tab']");
	if ($tab.length > 0) {
	// remove leading url if it exists. (copy of bootstrap url stripper)
	var href = $tab.attr("href").replace(/.*(?=#[^\s]+$)/, '');
	// remove tab id to get the index
	var index = href.replace("#tab-" + tabsetId + "-", "");
	existingTabIds.push(Number(index));
	}
	});

)

I originally changed this logic because, for a short period, we were working around a BS4 regression that dropped support for dropdown items wrapped in <li>. That regression has since been fixed and is now included with bs_theme(version = 4), so this change in logic is no longer needed