DoS via custom post type for sysconsole plugin readers

Mattermost versions 10.2.0, 9.11.x <= 9.11.5, 10.0.x <= 10.0.3, 10.1.x <= 10.1.3 fail to properly validate post types, which allows attackers to deny service to users with the sysconsole_read_plugins permission via creating a post with the custom_pl_notification type and specific props.