Revamp action_mailer config

`registrant_url` setting in `application.yml` is misnamed. In fact,
it has nothing to do with Registrant Portal. The intention is to
provide default URL options for `action_mailer`, therefore
`registrant_url` is removed, and new settings are introduced instead:

- `action_mailer_default_protocol`
- `action_mailer_default_host`
- `action_mailer_default_port`

config/initializers/figaro.rb

@@ -11,4 +11,5 @@ Figaro.require_keys(%w[
   legal_documents_dir
   bank_statement_import_dir
   time_zone
+  action_mailer_default_host
 ])