Send acceptance, rejection, and waitlist decision emails

[balebbae]

Now that we have a mailer client (internal/mailer/), add support for sending decision emails to applicants: acceptance, rejection, and waitlist.

Notes

• Follow the existing pattern used by SendQREmail / SendWalkInAcceptedEmail / SendWalkInQueuedEmail in internal/mailer/mailer.go, sendgrid.go, and smtp.go
• Add HTML templates under internal/mailer/template/ (e.g. accepted.html, rejected.html, waitlisted.html)
• Use HACKATHON_NAME for the event name in subjects/bodies (falls back to mailer.DefaultHackathonName), consistent with the other emails
• Wire up sending from the appropriate admin/superadmin application status-change flow