Clean up flash display in views (#2336)

* Add flashes partial to DRY up admin/auth layouts

* Further consolidate flash messages

app/helpers/flashes_helper.rb

+# frozen_string_literal: true
+
+module FlashesHelper
+  def user_facing_flashes
+    flash.to_hash.slice('alert', 'error', 'notice', 'success')
+  end
+end

app/views/application/_flashes.html.haml

+- user_facing_flashes.each do |key, value|
+  .flash-message{class: key}
+    %strong= value

app/views/layouts/admin.html.haml

@@ -13,13 +13,7 @@
       .content
         %h2= yield :page_title
 
-        - if flash[:notice]
-          .flash-message.notice
-            %strong= flash[:notice]
-
-        - if flash[:alert]
-          .flash-message.alert
-            %strong= flash[:alert]
+        = render 'flashes'
 
         = yield
 

app/views/layouts/auth.html.haml

@@ -9,13 +9,7 @@
           = image_tag 'logo.png'
 
     .form-container
-      - if flash[:notice]
-        .flash-message.notice
-          %strong= flash[:notice]
-
-      - if flash[:alert]
-        .flash-message.alert
-          %strong= flash[:alert]
+      = render 'flashes'
 
       = yield