Only display user-facing flashes

We sometimes use the flash for things such as
"sign up" in order to send data to analytics systems
such as Mixpanel.

We've then ended up doing the following on a number of projects.

lib/suspenders/app_builder.rb

@@ -135,7 +135,8 @@ def create_partials_directory
     end
 
     def create_shared_flashes
-      copy_file '_flashes.html.erb', 'app/views/application/_flashes.html.erb'
+      copy_file "_flashes.html.erb", "app/views/application/_flashes.html.erb"
+      copy_file "flashes_helper.rb", "app/helpers/flashes_helper.rb"
     end
 
     def create_shared_javascripts

templates/_flashes.html.erb

@@ -1,6 +1,6 @@
 <% if flash.any? %>
   <div id="flash">
-    <% flash.each do |key, value| -%>
+    <% user_facing_flashes.each do |key, value| -%>
       <div class="flash-<%= key %>"><%= value %></div>
     <% end -%>
   </div>

templates/flashes_helper.rb

@@ -0,0 +1,5 @@
+module FlashesHelper
+  def user_facing_flashes
+    flash.to_hash.slice("alert", "error", "notice", "success")
+  end
+end