Move hiding logic to stimulus controller

This ensures morphed flashes hide like other flashes (eg in bulk order actions). I wanted to write a spec to prove it, but Capybara doesn't support mocking setTimeout and I didn't want to use sleep.

I've made it optional because this controller is shared with the shop frontend ([supposedly](5ef34347a3), although angular seems to override it).

app/assets/javascripts/admin/spree/base.js.erb

@@ -32,9 +32,6 @@ jQuery(function($) {
     });
   }
 
-  // Make flash messages dissapear
-  setTimeout('$(".flash").fadeOut()', 5000);
-
   // Highlight hovered table column
   $('table tbody tr td.actions a').hover(function(){
     var tr = $(this).closest('tr');