app/views/application/_flash_messages.html.erb
<div id="flashes">
<% flash.each do |name, message| %>
<div class="flash flash-<%= name %>">
<span class='font-weight-bold'><%= name.titleize %></span>
<ul class='mb-0'><% Array(message).each do |m| %><li><%= m %> <% end %></li></ul>
</div>
<% end %>
</div>