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).

private/email-templates/reset-password.txt

@@ -0,0 +1,10 @@
+Hey, {{firstName}}!
+
+A password reset has been requested for the account related to this email address ({{emailAddress}}).
+
+To reset the password, visit the following link:
+
+[Reset Password]({{resetUrl}})
+
+Cheers,
+{{applicationName}} Team