[MM-41185]: new API to mark thread as unread (#19951)

* [MM-41185]: new API to mark thread as unread

Until now we mark a thread as read/unread by sending a timestamp to the
server. This created some issues when marking a thread as unread from a
post. We needed to send the post.create_at - 1 as a timestamp but we
didn't do so consistently.

This commit adds a new API to set a thread as unread by post id.
Making all clients agnostic of the timestamp, and thus solving consistency issues.

Endpoint: /api/v4/users/{user_id}/teams/{team_id}/threads/{thread_id}/set_unread/{post_id}

* Updates client.go adding SetThreadUnreadByPostId

* Guards endpoint behind read channel permission

* Returns status 400 if post_id not belong in thread

* Root post as post_id should be permitted

model/client4.go

@@ -7920,6 +7920,18 @@ func (c *Client4) UpdateThreadsReadForUser(userId, teamId string) (*Response, er
 	return BuildResponse(r), nil
 }
 
+func (c *Client4) SetThreadUnreadByPostId(userId, teamId, threadId, postId string) (*ThreadResponse, *Response, error) {
+	r, err := c.DoAPIPost(fmt.Sprintf("%s/set_unread/%s", c.userThreadRoute(userId, teamId, threadId), postId), "")
+	if err != nil {
+		return nil, BuildResponse(r), err
+	}
+	defer closeBody(r)
+	var thread ThreadResponse
+	json.NewDecoder(r.Body).Decode(&thread)
+
+	return &thread, BuildResponse(r), nil
+}
+
 func (c *Client4) UpdateThreadReadForUser(userId, teamId, threadId string, timestamp int64) (*ThreadResponse, *Response, error) {
 	r, err := c.DoAPIPut(fmt.Sprintf("%s/read/%d", c.userThreadRoute(userId, teamId, threadId), timestamp), "")
 	if err != nil {