FEATURE: support DISCOURSE_SMTP_FORCE_TLS option (#11733)

Background: RFC 8314 3.3 asks that:

clients and servers SHOULD implement both STARTTLS on
port 587 and Implicit TLS on port 465

Discourse currently cannot be configured this way.
With this patch, it's possible to set
DISCOURSE_SMTP_FORCE_TLS=true to use implicit TLS on port 465

config/discourse_defaults.conf

@@ -92,6 +92,9 @@ smtp_enable_start_tls = true
 # to disable, set to 'none'
 smtp_openssl_verify_mode =
 
+# force implicit TLS as per RFC 8314 3.3
+smtp_force_tls = false
+
 # load MiniProfiler in production, to be used by developers
 load_mini_profiler = true
 

config/environments/production.rb

@@ -37,6 +37,10 @@ Discourse::Application.configure do
 
     settings[:openssl_verify_mode] = GlobalSetting.smtp_openssl_verify_mode if GlobalSetting.smtp_openssl_verify_mode
 
+    if GlobalSetting.smtp_force_tls
+      settings[:tls] = true
+    end
+
     config.action_mailer.smtp_settings = settings.reject { |_, y| y.nil? }
   else
     config.action_mailer.delivery_method = :sendmail