Typescript migration:

* Refactoring of scheduler and jobs to use promises.
* Add "failed" state for tasks.

server/jobs/MailQueueJob.ts

@@ -0,0 +1,22 @@
+import Logger from "../logger"
+import MailService from "../services/mailService"
+
+export default {
+    name: 'MailQueueJob',
+    description: 'Send pending emails (up to 5 attempts in case of failures).',
+
+    run: (): Promise<void> => {
+        return new Promise<void>(
+            (resolve, reject) => {
+                MailService.sendPendingMails((err: any): void => {
+                    if (err) {
+                        Logger.tag('mail', 'queue').error('Error sending pending mails:', err);
+                        return reject(err);
+                    }
+
+                    resolve();
+                });
+            }
+        )
+    }
+}