Emailer: refactor, support bulk sending.

[fuisl]

Emailer refactor

This PR introduce class Emailer that will send email templates in bulks.

Usage

1. Initialize an Emailer Object with path to template and qrcode path (use relative path).

(since this emailer is being used for sending out digital tickets, these information is fixed and neccessary. If you wish to use this emailer for other purposes, override the content() method for changing the content of the email.)

2. Login to email service
   At this point, you can set test_email field to send out a test email to make sure that the system is working.

passkey = os.getenv("MAIL_PASSKEY")
emailer_obj.login(email, passkey, test_email="[email protected]")

3. Send ticket
   You can either use send_one() or send_many(). Doctrings are included, explore the code for more information.

emailer_obj.send_one(info)  # dict
emailer_obj.send_many(infos)  # list of dicts

Do notice the contents of the input dictionary, the embedded content should be filled.

Handle exceptions

Some exceptions will be raised. (e.g. SMTPRecipentsRefused, etc.) and theses situations are not covered by the system. Should have a mechanism to check if ticket sent successfully. @nhientruong04