config: use alternate delivery methods to configure ActionMailer

Previously `SENDINBLUE_BALANCING` was used only when
`SENDINBLUE_ENABLED` was *disabled* (otherwise only SendInBlue was ever
used).

This commit:

- Ensure that `SENDINBLUE_BALANCING` is used only when SendInBlue is
  *enabled* (which is more intuitive).
- Make it easier to add other delivery methods.

app/lib/mailtrap/smtp.rb

@@ -0,0 +1,4 @@
+module Mailtrap
+  class Smtp < ::Mail::SMTP
+  end
+end

app/lib/sendinblue/smtp.rb

@@ -0,0 +1,4 @@
+module Sendinblue
+  class Smtp < ::Mail::SMTP
+  end
+end