Make Devise::Mailer inherit from ApplicationMailer

This way we remove duplication in the `from:` proc, the helpers, and the
methods we're about to write.

config/initializers/devise.rb

@@ -14,11 +14,15 @@ Devise.setup do |config|
   # Configure the e-mail address which will be shown in Devise::Mailer,
   # note that it will be overwritten if you use your own mailer class
   # with default "from" parameter.
-  config.mailer_sender = proc { "'#{Setting["mailer_from_name"]}' <#{Setting["mailer_from_address"]}>" }
+  # We're not setting it here because it's set by the ApplicationMailer class
+  # config.mailer_sender = "please-change-me-at-config-initializers-devise@example.com"
 
   # Configure the class responsible to send e-mails.
   config.mailer = "DeviseMailer"
 
+  # Configure the parent class responsible to send e-mails.
+  config.parent_mailer = "ApplicationMailer"
+
   # ==> ORM configuration
   # Load and configure the ORM. Supports :active_record (default) and
   # :mongoid (bson_ext recommended) by default. Other ORMs may be