Modify admin layout to only manage tenants and admins

We only want to render the account link and login items in the header.
And we want only render the Multitenancy and Administrators sections in
the admin sidebar.

We include the administrators management so it's possible to give
permissions to other users to manage tenants.

In order to restrict access to other sections by typing the URL or
following a link, we're only enabling the rest of the routes when we
aren't in the multitenancy management mode.

spec/components/layout/subnavigation_component_spec.rb

@@ -0,0 +1,10 @@
+require "rails_helper"
+
+describe Layout::SubnavigationComponent do
+  it "is not rendered when multitenancy_management_mode is enabled" do
+    allow(Rails.application.config).to receive(:multitenancy_management_mode).and_return(true)
+    render_inline Layout::SubnavigationComponent.new
+
+    expect(page).not_to be_rendered
+  end
+end