[NO-TICKET] Fix preview mailers index

[adessy]

The list of preview mailers at localhost:4000/rails/mailers was broken because the newly introduced PreviewMailer interfered with the lazy loading of mailer previews.

Here's how ActionMailer::Preview determines the list of previews:

def all
  load_previews if descendants.empty?
  descendants
end

def load_previews
  if preview_path
    Dir["#{preview_path}/**/*_preview.rb"].sort.each { |file| require_dependency file }
  end
end

The load_previews method is only triggered if descendants is empty. However, since PreviewMailer was defined in an autoloaded path and is a subclass of ActionMailer::Preview, it was loaded too early and short-circuited the loading of the other mailer previews.

The fix consists of moving the PreviewMailer class to the same folder as the other mailer previews. The downside is that now it needs to be required explicitly in the previewers that use it.

Changelog

Technical

• Fix preview mailers index