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/authentication.lisp

@@ -78,6 +78,8 @@ and a retry"
     (ldap-entry->user ldap-entry)))
 
 (defun find-user-by-dn (dn)
+  "Look up the user with the given DN in the LDAP database, returning an
+instance of `user'"
   (with-ldap ()
     (let ((have-results
             (handler-case