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/system/management_spec.rb

@@ -1,38 +0,0 @@
-require "rails_helper"
-
-describe "Management" do
-  let(:user) { create(:user) }
-  before { Setting["org_name"] = "CONSUL" }
-
-  scenario "Should show admin menu if logged user is admin" do
-    create(:administrator, user: user)
-    login_as(user)
-
-    visit root_path
-    click_link "Menu"
-    click_link "Management"
-
-    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
-
-  scenario "Should not show admin menu if logged user is manager" do
-    create(:manager, user: user)
-    login_as(user)
-    visit root_path
-
-    click_link "Menu"
-    click_link "Management"
-
-    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