Make count message required

The count message always exists (since
7b534bf). This PR
makes the markup for it required, instead of giving the option to add it with JS since the user needs to be informed
of the limit in the number of characters they can type in even if JS doesn’t run.

The count message DOM element is now defined at the top of the module with the other required DOM elements.
We check whether the textarea is present before trying to its id to find the count message - the check prevents a
DOM search error for `$textarea.id` if the script is run without component markup being present present on the
page. Alternatively we could define the count message in `init()` after doing a check for textarea which would
require another check for it and wouldn’t make it as clear that it’s required as when placed at the top of module.

```
CharacterCount.prototype.init = function () {
  var $module = this.$module
  var $textarea = this.$textarea

  if (!$textarea) {
    return
  }

  this.$countMessage = $module.querySelector('[id=' + this.$textarea.id + '-info]')

  if (!$countMessage) {
    return
  }
```

src/govuk/components/character-count/character-count.js

@@ -5,6 +5,7 @@ import '../../vendor/polyfills/Element/prototype/classList'
 function CharacterCount ($module) {
   this.$module = $module
   this.$textarea = $module.querySelector('.govuk-js-character-count')
+  if (this.$textarea) this.$countMessage = $module.querySelector('[id=' + this.$textarea.id + '-info]')
 }
 
 CharacterCount.prototype.defaults = {
@@ -17,7 +18,9 @@ CharacterCount.prototype.init = function () {
   // Check for module
   var $module = this.$module
   var $textarea = this.$textarea
-  if (!$textarea) {
+  var $countMessage = this.$countMessage
+
+  if (!$textarea || !$countMessage) {
     return
   }
 
@@ -132,7 +135,7 @@ CharacterCount.prototype.checkIfValueChanged = function () {
 CharacterCount.prototype.updateCountMessage = function () {
   var countElement = this.$textarea
   var options = this.options
-  var countMessage = this.countMessage
+  var countMessage = this.$countMessage
 
   // Determine the remaining number of characters/words
   var currentLength = this.count(countElement.value)