use accessible autocomplete instead of select_with_search

Following an accessibility audit, the
select_with_search component was found not to meet
WCAG requirements - "Users should be able to
access this list using a keyboard or other input
methods, with each item being announced along with
a count of the total number of options."

It seems like on Safari Voice Over would not
read out the selected option at all, and in other
browsers would not read out the total count of
options.

I'm replacing the component with the autocomplete
component, which does not use the `Choices`
library and is based on the alphagov accessible
autocomplete, and seems to provide the same
functionality and the required Voice Over
behaviour. There are accessibility issues when
using this component with the `multiselect` option
(as Whitehall do)[1], but shouldn't be when just
using it as a combobox.

I've also had to change one of our feature specs,
because using the javascript helpers changes how
the `select` function works (it's designed for the
Choices library). We don't actually need to fill
in the form so I've just removed that step.

[1]
https://drive.google.com/file/d/1BgR9Tkpr9nEYoU3mrUwue306XjLAsZ7B/view?usp=sharing
(when the selected options appear below the combobox they are not
announced, and not keyboard navigable)

lib/engines/content_block_manager/app/views/content_block_manager/content_block/shared/_form.html.erb

@@ -31,19 +31,20 @@
     } %>
   <% end %>
 
-  <%= render "components/select_with_search", {
+  <%= render "components/autocomplete", {
     id: "#{parent_class}_lead_organisation",
     name: "content_block/edition[organisation_id]",
-    label:  "Lead organisation",
-    include_blank: true,
-    options: taggable_organisations_container.map do |name, id|
-      {
-        text: name,
-        value: id,
-        selected: id == @form.content_block_edition.edition_organisation&.organisation_id,
-      }
-    end,
-    error_message: errors_for_input(@form.content_block_edition.errors, "lead_organisation".to_sym),
+    label: {
+      text: "Lead organisation",
+    },
+    select: {
+      multiple: false,
+      options: [""] + taggable_organisations_container,
+      selected: @form.content_block_edition.edition_organisation&.organisation_id,
+    },
+    autocomplete_configuration_options: {
+      showAllValues: true,
+    },
   } %>
 
   <%= render "govuk_publishing_components/components/textarea", {

lib/engines/content_block_manager/features/edit_object.feature

@@ -131,7 +131,7 @@ Feature: Edit a content object
   @javascript
   Scenario: GDS editor can preview a host document
     When I revisit the edit page
-    And I fill out the form
+    And I save and continue
     Then I am shown where the changes will take place
     And I see the rollup data for the dependent content
     When I click on the first host document

lib/engines/content_block_manager/features/step_definitions/content_block_manager_steps.rb

@@ -627,7 +627,7 @@ def should_show_edit_form_for_email_address_content_block(document_title, email_
   page.switch_to_window(page.windows.last)
   assert_text "Preview email address"
   assert_text "Instances: 1"
-  assert_text "Email address: [email protected]"
+  assert_text "Email address: #{@email_address}"
   within_frame "preview" do
     assert_text @current_host_document["title"]
   end
@@ -642,7 +642,7 @@ def should_show_edit_form_for_email_address_content_block(document_title, email_
 Then("I should see the content of the linked page") do
   within_frame "preview" do
     assert_text "other page"
-    assert_text "[email protected]"
+    assert_text @email_address
   end
 end