Improve date picker UI

- Set first day of week to Monday
- Enable year and month selectors

#590

app/assets/javascripts/datepicker.js

@@ -0,0 +1,21 @@
+(function () {
+    $.datepicker.setDefaults({
+        changeMonth: true,
+        changeYear: true,
+        duration: 'fast',
+        firstDay: 1, // Monday
+        dateFormat: 'yy-mm-dd',
+    });
+
+    function attachDatePicker() {
+        var dateFields = $('.datepicker');
+        dateFields.datepicker();
+    }
+
+    // For turbolinks
+    document.addEventListener('page:change', function () {
+        attachDatePicker();
+    });
+
+    attachDatePicker();
+})();