Attach PDF ticket to confirmation email

[icewindow]

Issue Description

When a user gets the confirmation mail that the ticket purchase was completed, they should also receive the ticket as an attached PDF file. See also #614.

Motivation

Sending a PDF file will make it somewhat obvious that this is something important to the user. We have had users who didn't know they received a QR code in their profile upon completion of their purchase and when time to check them into the vent came, were not immediately able to produce the QR code due to them not having a working internet connection or them not knowing their login details. Many people have their emails synced up to their mobile devices, so this provides a backup solution in case the user is unable to access their profile.

Considerations

How should updated PDFs be handled?

If the PDF file is updated for any reason, should the user get a follow-up email with the updated document attached? Should the user just be notified and prompted to download the updated file via their profile? Should the change not trigger any email?

The best solution is probably the first one, sending the updated PDF as a follow-up email. However, if multiple PDF files needed to be changed, this could strain the server resources. A queuing system might be needed.

Compatibility with already existing mail handling

The mailing system currently uses templates to build the email. Can this system cope with attachments? Can a middleware be added to the mail processing chain to add attachments?

Possible implementation

The PDF should be already generated at the time the confirmation mail is sent. The handler which sends the mail will need to read in the file and attach it to the email.