app/views/application/_flash.html.erb
<% unless flash.empty? %>
<div class="container">
<% flash.select { |_, msg| msg.is_a? String }.each do |type, msg| %>
<div class="alert <%= bootstrap_class_for_flash type %> alert-dismissable fade show mt-3">
<%= msg %>
<button class="close" data-dismiss="alert">x</button>
</div>
<% end %>
</div>
<% end %>