- Refactoring of the mail sending part. Mail queue (table 'queued') is now used also for all test emails.

- More options how to send test emails.
- Fixed problems with pausing a campaign (#593)
- Started rework of transactional sender of templates (#606), however this contains functionality regression at the moment because it does not interpret templates as HBS. It needs HBS option for templates as described in https://github.com/Mailtrain-org/mailtrain/issues/611#issuecomment-502345227

TODO:
- detect sending errors connected to not able to contact the mailer and pause/retry campaing and queued sending - don't mark the recipients as BOUNCED
- add FAILED campaign state and fall into it if sending to campaign consistently fails (i.e. the error with sending is not temporary)
- if the same happends for queued email, delete the message

server/lib/subscription-mail-helpers.js

@@ -139,7 +139,7 @@ async function _sendMail(list, email, template, locale, subjectKey, relativeUrls
     try {
         if (list.send_configuration) {
             const mailer = await mailers.getOrCreateMailer(list.send_configuration);
-            await mailer.sendTransactionalMail({
+            await mailer.sendTransactionalMailBasedOnTemplate({
                 to: {
                     name: getDisplayName(flds, subscription),
                     address: email