[feature/new-tz-handling] Preselect a timezone in registration.

Use Javascript to obtain client's timezone offset and select the
first timezone in our list with that offset.

Changes for prosilver only. The javascript file should be shared
between styles.

PHPBB3-9558

phpBB/styles/prosilver/template/timezone.js

@@ -0,0 +1,54 @@
+function phpbb_preselect_tz_select()
+{
+	var selector = document.getElementsByClassName('tz_select')[0];
+	if (selector.value)
+	{
+		return;
+	}
+	// The offset returned here is in minutes and negated.
+	// http://www.w3schools.com/jsref/jsref_getTimezoneOffset.asp
+	var offset = (new Date()).getTimezoneOffset();
+	if (offset < 0)
+	{
+		var sign = '+';
+		offset = -offset;
+	}
+	else
+	{
+		var sign = '-';
+	}
+	var minutes = offset % 60;
+	var hours = (offset - minutes) / 60;
+	if (hours < 10)
+	{
+		hours = '0' + hours.toString();
+	}
+	else
+	{
+		hours = hours.toString();
+	}
+	if (minutes < 10)
+	{
+		minutes = '0' + minutes.toString();
+	}
+	else
+	{
+		minutes = minutes.toString();
+	}
+	var prefix = 'GMT' + sign + hours + ':' + minutes;
+	var prefix_length = prefix.length;
+	for (var i = 0; i < selector.options.length; ++i)
+	{
+		var option = selector.options[i];
+		if (option.value.substring(0, prefix_length) == prefix)
+		{
+			// Firefox scrolls the selector only to put the option into view;
+			// for negative-offset timezones, this means the first timezone
+			// of a particular offset will be the bottom one, and selected,
+			// with all other timezones not visible. Not much can be done
+			// about that here unfortunately.
+			option.selected = true;
+			break;
+		}
+	}
+}