Fixes public directory structure

Some of the changes made before meant that js files were being compiled
and placed in a `govuk` directory. This meant that the app could not
find the js files eg. `govuk/all.js`

Current & desired folder structure for `public` when serving up the review app:

public
├── components
│   ├── accordion
│   ├── button
│   ├── character-count
│   ├── checkboxes
│   ├── details
│   ├── error-summary
│   ├── header
│   ├── radios
│   └── tabs
├── vendor
│   └── polyfills
├── all.js
├── app-ie8.css
├── app-legacy-ie8.css
├── app-legacy.css
├── app.css
└── common.js

tasks/gulp/compile-assets.js

@@ -28,6 +28,17 @@ const postcsspseudoclasses = require('postcss-pseudo-classes')({
 // check if destination flag is dist
 const isDist = taskArguments.destination === 'dist' || false
 
+// Set the destination
+const destinationPath = function () {
+  if (taskArguments.destination === 'dist') {
+    return taskArguments.destination
+  } else if (taskArguments.destination === 'public') {
+    return taskArguments.destination
+  } else {
+    return `${taskArguments.destination}/govuk/`
+  }
+}
+
 const errorHandler = function (error) {
   // Log the error to the console
   console.error(error.message)
@@ -142,7 +153,6 @@ gulp.task('scss:compile', () => {
 gulp.task('js:compile', () => {
   // for dist/ folder we only want compiled 'all.js' file
   let srcFiles = isDist ? configPaths.src + 'all.js' : configPaths.src + '**/*.js'
-  let destination = isDist ? taskArguments.destination : taskArguments.destination + '/govuk/'
 
   return gulp.src([
     srcFiles,
@@ -166,5 +176,5 @@ gulp.task('js:compile', () => {
       })
     ))
     .pipe(eol())
-    .pipe(gulp.dest(destination))
+    .pipe(gulp.dest(destinationPath))
 })