feat(panettone): Add functions to send email notifications

Add a new package to panettone, :panettone.email with functions to send
email notifications to users through the SMTP relay on whitby,
respecting the value of `enable_email_notifications` on the
user_settings table.

Change-Id: Ia4ec65965abda06f1fadb178143d66bb8eae6482
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2804
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: tazjin <mail@tazj.in>

web/panettone/src/email.lisp

@@ -0,0 +1,48 @@
+(in-package :panettone.email)
+(declaim (optimize (safety 3)))
+
+(defvar *smtp-server* "localhost"
+  "The host for SMTP connections")
+
+(defvar *smtp-server-port* 2525
+  "The port for SMTP connections")
+
+(defvar *notification-from* "tvlbot@tazj.in"
+  "The email address to send email notifications from")
+
+(defvar *notification-from-display-name* "Panettone"
+  "The Display Name to use when sending email notifications")
+
+(defvar *notification-subject-prefix* "[panettone]"
+  "String to prefix all email subjects with")
+
+(defun send-email-notification (&key to subject message)
+  "Sends an email to TO with the given SUBJECT and MESSAGE, using the current
+values of `*smtp-server*', `*smtp-server-port*' and `*email-notification-from*'"
+  (let ((subject (if *notification-subject-prefix*
+                     (format nil "~A ~A"
+                             *notification-subject-prefix*
+                             subject)
+                     subject)))
+    (cl-smtp:send-email
+     *smtp-server*
+     *notification-from*
+     to
+     subject
+     message
+     :port *smtp-server-port*
+     :display-name *notification-from-display-name*)))
+
+(defun user-has-email-notifications-enabled-p (dn)
+  "Returns T if the user with the given DN has enabled email notifications"
+  (enable-email-notifications-p (settings-for-user dn)))
+
+(defun notify-user (dn &key subject message)
+  "Sends an email notification to the user with DN with the given SUBJECT and
+  MESSAGE, iff that user has not disabled email notifications"
+  (when (user-has-email-notifications-enabled-p dn)
+    (when-let ((user (find-user-by-dn dn)))
+      (send-email-notification
+       :to (mail user)
+       :subject subject
+       :message message))))