Convert text inputs on ServiceUpdateEmailBranding

Includes changes to templates that use this form
and associated tests.

app/assets/javascripts/colourPreview.js

@@ -8,9 +8,9 @@
 
     this.start = component => {
 
-      this.$input = $('input', component);
+      this.$input = $(component);
 
-      $(component).append(
+      this.$input.closest('.govuk-form-group').append(
         this.$preview = $('<span class="textbox-colour-preview"></span>')
       );
 

app/assets/stylesheets/govuk-frontend/extensions.scss

@@ -48,3 +48,7 @@ $govuk-grid-widths: (
   seven-eighths: 87.5%,
   full: 100%
 );
+
+.govuk-input--width-6 {
+  max-width: 14ex;
+}