Hackathon: Post Reminders (#20555)

This PR adds the post reminder backend work.

We add a new API endpoint via which a user can set a reminder for a post. An ephemeral message will be sent down the line to let the user know about the action. And then after the time is over, the system admin bot will send a DM message to the user about the reminder post.

app/channels.go

@@ -86,6 +86,9 @@ type Channels struct {
 
 	dndTaskMut sync.Mutex
 	dndTask    *model.ScheduledTask
+
+	postReminderMut  sync.Mutex
+	postReminderTask *model.ScheduledTask
 }
 
 func init() {