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.

api4/post.go

@@ -34,6 +34,8 @@ func (api *API) InitPost() {
 	api.BaseRoutes.Post.Handle("", api.APISessionRequired(updatePost)).Methods("PUT")
 	api.BaseRoutes.Post.Handle("/patch", api.APISessionRequired(patchPost)).Methods("PUT")
 	api.BaseRoutes.PostForUser.Handle("/set_unread", api.APISessionRequired(setPostUnread)).Methods("POST")
+	api.BaseRoutes.PostForUser.Handle("/reminder", api.APISessionRequired(setPostReminder)).Methods("POST")
+
 	api.BaseRoutes.Post.Handle("/pin", api.APISessionRequired(pinPost)).Methods("POST")
 	api.BaseRoutes.Post.Handle("/unpin", api.APISessionRequired(unpinPost)).Methods("POST")
 }
@@ -842,6 +844,36 @@ func setPostUnread(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 }
 
+func setPostReminder(c *Context, w http.ResponseWriter, r *http.Request) {
+	c.RequirePostId().RequireUserId()
+	if c.Err != nil {
+		return
+	}
+
+	if c.AppContext.Session().UserId != c.Params.UserId && !c.App.SessionHasPermissionToUser(*c.AppContext.Session(), c.Params.UserId) {
+		c.SetPermissionError(model.PermissionEditOtherUsers)
+		return
+	}
+	if !c.App.SessionHasPermissionToChannelByPost(*c.AppContext.Session(), c.Params.PostId, model.PermissionReadChannel) {
+		c.SetPermissionError(model.PermissionReadChannel)
+		return
+	}
+
+	var reminder model.PostReminder
+	if jsonErr := json.NewDecoder(r.Body).Decode(&reminder); jsonErr != nil {
+		c.SetInvalidParam("target_time")
+		return
+	}
+
+	appErr := c.App.SetPostReminder(c.Params.PostId, c.Params.UserId, reminder.TargetTime)
+	if appErr != nil {
+		c.Err = appErr
+		return
+	}
+
+	ReturnStatusOK(w)
+}
+
 func saveIsPinnedPost(c *Context, w http.ResponseWriter, isPinned bool) {
 	c.RequirePostId()
 	if c.Err != nil {