Add a calendar attachment to confirmation emails

[Ardnived]

Calendar attachments need to be created in rails, and then sent to Brevo (aka Sendinblue) to be attached to our emails.

• https://stackoverflow.com/questions/4708056/how-to-create-ics-in-rails-and-send-it-as-a-attchment-in-a-mail
• https://help.brevo.com/hc/en-us/articles/4402811730962-Can-I-add-an-attachment-to-a-transactional-email-
• https://developers.brevo.com/reference/sendtransacemail

[Ardnived]

Hopefully at some point, Montrose, our recurrence gem, will support serializing recurrences in the ical format. https://github.com/rossta/montrose/issues/26