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/stylesheets/components/textbox.scss

@@ -62,11 +62,13 @@
 
 .textbox-colour-preview {
   @include media(desktop) {
-    width: 34px;
-    height: 34px;
+    width: 38px;
+    height: 38px;
     margin-left: 5px;
-    border: 2px solid #0B0C0C;
+    border-radius: 50%;
+    box-shadow: inset 0 0 0 1px rgba($black, 0.2);
     display: inline-block;
     vertical-align: top;
+    transition: background 0.3s ease-out;
   }
 }