[cloud-workers, isomorphic-core] Implement the SendRemindersWorker

Summary:
Process reminders metadata and send the reminder email if there
haven't been any replies. Checking for replies involves checking
each of the folders in `folderImapNames` for messages that are in
reply to the messages in `messageIdHeaders`.

Test Plan: manual

Reviewers: khamidou, juan, evan

Reviewed By: evan

Differential Revision: https://phab.nylas.com/D3958

packages/isomorphic-core/src/models/account.js

@@ -130,6 +130,9 @@ module.exports = (sequelize, Sequelize) => {
         }
         if (this.provider === 'gmail') {
           const {xoauth2} = this.decryptedCredentials();
+          if (!xoauth2) {
+            throw new Error("Missing XOAuth2 Token")
+          }
           const {imap_username} = this.connectionSettings;
           const token = this.bearerToken(xoauth2);
           config.auth = { user: imap_username, xoauth2: token }