feat: sign up, entered, and voted email automations

[siobh9]

implementation thoughts for email automation (from research in #3005)

types of emails: https://docs.google.com/spreadsheets/d/1TbklqpK-iwlySUDXiR92wNQgyTLQxmC1B1Z_0Avyr5I/edit?usp=sharing (this ticket is for the first 3: Sign-Up Confirmation, Contest Entry Confirmation. Voting Start Notification (entrants))

what setting up automation looks like in brevo's no-code tool:

docs for sending those custom events via their REST API: https://tracker-doc.brevo.com/docs/track-custom-events-1

[siobh9]

for this ticket, @nakedfool and I will need to be able to log into Brevo - @pagliachita mind sharing that with us?