Wait for AJAX after steps defined in SharedActiveTab

Signed-off-by: Rémy Coutable <remy@rymai.me>

features/steps/shared/active_tab.rb

 module SharedActiveTab
   include Spinach::DSL
+  include WaitForAjax
+
+  after do
+    wait_for_ajax if javascript_test?
+  end
 
   def ensure_active_main_tab(content)
     expect(find('.layout-nav li.active')).to have_content(content)