Fix missing jQuery

Somewhere in the many rebases the bits of code that add jQuery to the production
Javascript got lost.

This caused Javascript to fail on all pages.

This commit re-adds that missing code.

gulpfile.babel.js

@@ -45,7 +45,7 @@ gulp.task('jquery', () => plugins.jquery.src({
   .pipe(gulp.dest(paths.dist + 'javascripts/'))
 );
 
-gulp.task('javascripts', () => gulp.src([
+gulp.task('javascripts', ['jquery'], () => gulp.src([
     paths.src + 'govuk_frontend_toolkit/javascripts/govuk/modules.js',
     paths.src + 'javascripts/highlightTags.js',
     paths.src + 'javascripts/main.js'
@@ -53,6 +53,9 @@ gulp.task('javascripts', () => gulp.src([
   .pipe(plugins.babel({
     presets: ['es2015']
   }))
+  .pipe(plugins.addSrc.prepend(
+    paths.dist + 'javascripts/jquery.custom.js'
+  ))
   .pipe(plugins.uglify())
   .pipe(plugins.concat('all.js'))
   .pipe(gulp.dest(paths.dist + 'javascripts/'))