Add custom autocomplete stylesheet

This is copied from the prototype and includes some overrides to make it
align to the NHS styles better.

app/assets/stylesheets/_autocomplete.scss

@@ -0,0 +1,44 @@
+// Ensure the autocomplete uses the correct typeface
+.autocomplete__wrapper {
+  font-family: $govuk-font-family;
+}
+
+.autocomplete__input {
+  font-family: inherit;
+}
+
+// Style the autocomplete if there’s an error
+.nhsuk-form-group--error {
+  .autocomplete__input {
+    border-color: $govuk-error-colour;
+  }
+
+  .autocomplete__input--focused {
+    border-color: $govuk-input-border-colour;
+  }
+}
+
+.autocomplete__dropdown-arrow-down {
+  // Ensure dropdown arrow can be clicked
+  // https://github.com/alphagov/accessible-autocomplete/issues/202
+  pointer-events: none;
+  // Ensure dropdown arrow can be seen
+  z-index: 0;
+}
+
+.autocomplete__input {
+  background-color: $color_nhsuk-white;
+}
+
+.autocomplete__option {
+  margin-bottom: 0;
+}
+
+.autocomplete__option-hint {
+  color: $nhsuk-secondary-text-color;
+
+  .autocomplete__option:hover &,
+  .autocomplete__option:focus & {
+    color: $color_nhsuk-grey-5;
+  }
+}

app/assets/stylesheets/application.scss

@@ -33,6 +33,7 @@ $color_app-dark-orange: darken(
 
 // Application components
 @import "action_list";
+@import "autocomplete";
 @import "button";
 @import "button-group";
 @import "card";