Show "Remove logo" buttons only when Appearance settings are persisted

This also adds the CarrierWave cache fields to the form as recommended
by https://github.com/carrierwaveuploader/carrierwave#making-uploads-work-across-form-redisplays
so that uploaded files aren't lost between form submits (i.e., when
there are errors on the form).

app/controllers/admin/appearances_controller.rb

@@ -51,7 +51,9 @@ class Admin::AppearancesController < Admin::ApplicationController
 
   # Only allow a trusted parameter "white list" through.
   def appearance_params
-    params.require(:appearance).permit(:title, :description, :logo,
-                                       :dark_logo, :light_logo, :updated_by)
+    params.require(:appearance).permit(
+      :title, :description, :logo, :logo_cache, :light_logo, :light_logo_cache,
+      :updated_by
+    )
   end
 end

app/views/admin/appearances/_form.html.haml

@@ -21,10 +21,12 @@
     = f.label :logo, class: 'control-label'
     .col-sm-10
       - if @appearance.logo?
-        = image_tag @appearance.logo, class: 'appearance-logo-preview'
-        %br
-        = link_to 'Remove logo', logo_admin_appearances_path, data: { confirm: "Logo will be removed. Are you sure?"}, method: :delete, class: "btn btn-remove btn-small remove-logo"
+        = image_tag @appearance.logo_url, class: 'appearance-logo-preview'
+        - if @appearance.persisted?
+          %br
+          = link_to 'Remove logo', logo_admin_appearances_path, data: { confirm: "Logo will be removed. Are you sure?"}, method: :delete, class: "btn btn-remove btn-small remove-logo"
         %hr
+      = f.hidden_field :logo_cache
       = f.file_field :logo, class: ""
       .hint
         Maximum logo size is 1MB, page optimized for logo size 640x360px
@@ -36,14 +38,15 @@
     = f.label :light_logo, 'Header logo', class: 'control-label'
     .col-sm-10
       - if @appearance.light_logo?
-        = image_tag @appearance.light_logo, class: 'appearance-light-logo-preview'
+        = image_tag @appearance.light_logo_url, class: 'appearance-light-logo-preview'
+        - if @appearance.persisted?
+          %br
+          = link_to 'Remove header logo', header_logos_admin_appearances_path, data: { confirm: "Header logo will be removed. Are you sure?"}, method: :delete, class: "btn btn-remove btn-small remove-logo"
+        %hr
+      = f.hidden_field :light_logo_cache
       = f.file_field :light_logo, class: ""
       .hint
         Maximum size is 1MB, page optimized for logo size 72x72px
-      -if @appearance.light_logo?
-        %br
-          = link_to 'Remove header logo', header_logos_admin_appearances_path, data: { confirm: "Header logos will be removed. Are you sure?"}, method: :delete, class: "btn btn-remove btn-small remove-logo"
-
 
   .form-actions
     = f.submit 'Save', class: 'btn btn-save'