MM-28249 Auto follow threads (#15878)

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/post.go

@@ -443,6 +443,12 @@ func (a *App) handlePostEvents(post *model.Post, user *model.User, channel *mode
 		return err
 	}
 
+	if *a.Config().ServiceSettings.ThreadAutoFollow && post.RootId != "" {
+		if err := a.Srv().Store.Thread().CreateMembershipIfNeeded(post.UserId, post.RootId); err != nil {
+			return err
+		}
+	}
+
 	if post.Type != model.POST_AUTO_RESPONDER { // don't respond to an auto-responder
 		a.Srv().Go(func() {
 			_, err := a.SendAutoResponseIfNecessary(channel, user)
@@ -1450,3 +1456,7 @@ func isPostMention(user *model.User, post *model.Post, keywords map[string][]str
 
 	return false
 }
+
+func (a *App) GetThreadMembershipsForUser(userId string) ([]*model.ThreadMembership, error) {
+	return a.Srv().Store.Thread().GetMembershipsForUser(userId)
+}