Refactor hex colour preview Javascript

This commit improves the code that previews a hex colour when setting up
or changing an email branding.

Specifically it:
- refactors the Javascript to conform to our patterns (module pattern,
  preprocessed with Gulp)
- makes the code work when there are multiple colour previews on one
  page

It also does some visual prettifying, because I couldn’t help myself…

app/assets/javascripts/colourPreview.js

@@ -0,0 +1,26 @@
+(function(Modules) {
+  "use strict";
+
+  let isSixDigitHex = value => value.match(/^#[0-9A-F]{6}$/i);
+  let colourOrWhite = value => isSixDigitHex(value) ? value : '#FFFFFF';
+
+  Modules.ColourPreview = function() {
+
+    this.start = component => {
+
+      this.$input = $('input', component);
+      this.$preview = $('.textbox-colour-preview', component);
+
+      this.$input
+        .on('change keyup', this.update)
+        .trigger('change');
+
+    };
+
+    this.update = () => this.$preview.css(
+      'background', colourOrWhite(this.$input.val())
+    );
+
+  };
+
+})(window.GOVUK.Modules);