Make autofocus textbox module

This commit adds a very small Javascript module to autofocus a textbox on page
load. It should only be used once per page.

It also adds a parameter to the textbox macro which adds the attribute to
trigger autofocus.

app/assets/javascripts/autofocus.js

@@ -0,0 +1,12 @@
+(function(Modules) {
+  "use strict";
+
+  Modules.Autofocus = function() {
+    this.start = function(component) {
+
+      $('input, textarea, select', component).eq(0).trigger('focus');
+
+    };
+  };
+
+})(window.GOVUK.Modules);

app/templates/components/textbox.html

@@ -1,5 +1,13 @@
-{% macro textbox(field, hint=False, highlight_tags=False, help_link=None, help_link_text=None, width='2-3') %}
-  <div class="form-group{% if field.errors %} error{% endif %}">
+{% macro textbox(
+  field,
+  hint=False,
+  highlight_tags=False,
+  autofocus=False,
+  help_link=None,
+  help_link_text=None,
+  width='2-3'
+) %}
+  <div class="form-group{% if field.errors %} error{% endif %}" {% if autofocus %}data-module="autofocus"{% endif %}>
     <label class="form-label" for="{{ field.name }}">
       {{ field.label }}
       {% if hint %}

gulpfile.babel.js

@@ -35,6 +35,7 @@ gulp.task('javascripts', () => gulp
     paths.npm + 'govuk_frontend_toolkit/javascripts/govuk/modules.js',
     paths.npm + 'govuk_frontend_toolkit/javascripts/govuk/selection-buttons.js',
     paths.src + 'javascripts/apiKey.js',
+    paths.src + 'javascripts/autofocus.js',
     paths.src + 'javascripts/highlightTags.js',
     paths.src + 'javascripts/main.js'
   ])