Move advanced search toggle button inside the form

Since forms are landmarks, screen reader users might navigate to the
form. But then they were going to find an empty form with no way to
toggle it.

Moving the button inside the form means screen reader users navigating
to the form will find the button to toggle it.

It also helps us simplifying the code; there's no need to use
data-attributes to communicate whether the form should be visible since
now we can easily use the button `aria-expanded` attribute.

We could further simplify the JavaScript if we used a CSS rule to
show/hide the form fields based on the toggle button `aria-expanded`
attribute. However, implementing the "slide" animation we use when
toggling the form with CSS is difficult and unreliable.

spec/components/shared/advanced_search_component_spec.rb

@@ -13,7 +13,7 @@ describe Shared::AdvancedSearchComponent, type: :component do
     it "hides the button to show the form" do
       render_inline component
 
-      expect(page).to have_button "Advanced search", visible: :hidden
+      expect(page.find("form")).to have_button "Advanced search", visible: :hidden
     end
   end