Respect WP's first day of week setting in schedule calendar

[MAILPOET-1995]

assets/js/src/newsletters/send/date_text.jsx

@@ -94,6 +94,7 @@ class DateText extends React.Component {
 
       $element.datepicker(_.extend({
         dateFormat: this.props.displayFormat,
+        firstDay: window.mailpoet_start_of_week,
         isRTL: false,
         onSelect: function onSelect(value) {
           that.onChange({

lib/Twig/Functions.php

@@ -123,6 +123,11 @@ class Functions extends AbstractExtension {
         array($this, 'isWoocommerceActive'),
         array('is_safe' => array('all'))
       ),
+      new TwigFunction(
+        'wp_start_of_week',
+        array($this, 'getWPStartOfWeek'),
+        array('is_safe' => array('all'))
+      ),
     );
   }
 
@@ -166,6 +171,10 @@ class Functions extends AbstractExtension {
     return WPFunctions::get()->getOption('date_format') ?: 'F j, Y';
   }
 
+  function getWPStartOfWeek() {
+    return WPFunctions::get()->getOption('start_of_week') ?: 0;
+  }
+
   function getMailPoetVersion() {
     return MAILPOET_VERSION;
   }

views/newsletters.html

@@ -17,6 +17,7 @@
     var mailpoet_server_time_zone = <%= json_encode(current_time_zone) %>;
     var mailpoet_schedule_time_of_day = <%= json_encode(schedule_time_of_day) %>;
     var mailpoet_date_display_format = "<%= wp_date_format() %>";
+    var mailpoet_start_of_week = "<%= wp_start_of_week() %>";
     var mailpoet_site_url = "<%= site_url %>";
     var mailpoet_date_storage_format = "Y-m-d";
     var mailpoet_tracking_enabled = <%= json_encode(tracking_enabled) %>;