add support for transactional html/text emails

- Add support for welcome email on user creation (both OAuth and password users)
- Add support for html/text templating on verify email.
- Add support for html/text templating on reset password.
- Add handlers for converting Handlebars templates to HTML and text (template agnostic).
- Add helper function sendEmail() for compiling and sending templates via Email.send() (SMTP).

imports/startup/server/accounts/on-create-user.js

@@ -1,7 +1,9 @@
 import { Accounts } from 'meteor/accounts-base';
+import sendWelcomeEmail from '../../../api/Users/server/send-welcome-email';
 
 Accounts.onCreateUser((options, user) => {
   const userToCreate = user;
   if (options.profile) userToCreate.profile = options.profile;
+  sendWelcomeEmail(options, user);
   return userToCreate;
 });