Allow for sending announcement mails with List-Unsubscribe:

Per https://datatracker.ietf.org/doc/html/rfc8058,
we send "announcement" mails with
  List-Unsubscribe: <${apiUrl}/unsubscribe/${userId}/${oneClickUnsubscribeToken}>
  List-Unsubscribe-Post: List-Unsubscribe=One-Click
and handle
  POST /api/unsubscribe/:user/:token => this unsubscribes
  GET  /api/unsubscribe/:user/:token => 302 /unsubscribe/:user/:token
  GET  /unsubscribe/:user/:token     => user-visible page with clickthrough confirmation

In this configuration, compatible MUAs will show an "unsubscribe" button
that, when clicked, will POST to the URL directly

Less-compatible MUAs (and scanners) will open the page directly
which will redirect to a click-though; interactive users will be able to
unsubscribe, scanners won't unsubscribe by accident

Nothing /actually/ sends non-reactive mails,
so this is never used at this time

Closes #854

packages/backend/src/models/UserProfile.ts

@@ -223,6 +223,11 @@ export class MiUserProfile {
 	})
 	public receiveAnnouncementEmail: boolean;
 
+	@Column('text', {
+		nullable: true,
+	})
+	public oneClickUnsubscribeToken: string | null;
+
 	@Column({
 		...id(),
 		nullable: true,