Move admin header partial to a component

This way it's easier to refactor it.

Note we're using `with_request_url` in the tests because the component
renders the locale switcher, which needs a URL in order to work. This
doesn't affect whether we're in the management section or not.

spec/components/layout/admin_header_component_spec.rb

@@ -0,0 +1,38 @@
+require "rails_helper"
+
+describe Layout::AdminHeaderComponent do
+  let(:user) { create(:user) }
+  before { Setting["org_name"] = "CONSUL" }
+
+  around do |example|
+    with_request_url("/") { example.run }
+  end
+
+  context "management section", controller: Management::BaseController do
+    it "shows the menu for administrators" do
+      create(:administrator, user: user)
+      sign_in(user)
+
+      render_inline Layout::AdminHeaderComponent.new(user)
+
+      expect(page).to have_link "Go back to CONSUL"
+      expect(page).to have_link "You don't have new notifications"
+      expect(page).to have_link "My content"
+      expect(page).to have_link "My account"
+      expect(page).to have_link "Sign out"
+    end
+
+    it "does not show the menu managers" do
+      create(:manager, user: user)
+      sign_in(user)
+
+      render_inline Layout::AdminHeaderComponent.new(user)
+
+      expect(page).to have_link "Go back to CONSUL"
+      expect(page).not_to have_content "You don't have new notifications"
+      expect(page).not_to have_content "My content"
+      expect(page).not_to have_content "My account"
+      expect(page).not_to have_content "Sign out"
+    end
+  end
+end