config: refactor email settings.

* Add `Danbooru.config.mail_delivery_method` option.
* Remove `Danbooru.config.aws_ses_enabled?` option.
* Replace `Danbooru.config.aws_ses_options` with `Danbooru.config.mail_settings`.

config/initializers/z_action_mailer.rb

@@ -1,9 +0,0 @@
-if Danbooru.config.aws_ses_enabled? && Rails.env == "production"
-  Rails.application.config.action_mailer.smtp_settings = {
-    :address => Danbooru.config.aws_ses_options[:smtp_server_name],
-    :user_name => Danbooru.config.aws_ses_options[:ses_smtp_user_name],
-    :password => Danbooru.config.aws_ses_options[:ses_smtp_password],
-    :authentication => :login,
-    :enable_starttls_auto => true
-  }
-end