Allow different custom images per tenant

Note this only affects images which can also be customized using the
administration interface; other images like `avatar_admin.png` must be
the same for all tenants. I think this is good enough for now, since the
images that can't be different are the images that aren't customized
that often, and if there's a need to use different images in a certain
CONSUL installation, it can be achieved by changing the code which
renders that image so it uses `image_path_for`.

app/helpers/application_helper.rb

@@ -53,6 +53,8 @@ module ApplicationHelper
 
     if image
       polymorphic_path(image)
+    elsif AssetFinder.find_asset(File.join(Tenant.subfolder_path, filename))
+      File.join(Tenant.subfolder_path, filename)
     else
       filename
     end