No question that ST needs a better UI. (They have a team working on it, but no projected timeline yet.) meanwhile, This particular app already exists. #3 under Safety and Security, Enhanced Autolock.
The following is current as of Sept 9, 2015. This is a list of the officially approved SmartApps that are available for installation through the SmartThings mobile app under the SmartApps tab. In each category, you will find a combination of community contributed & SmartThings developed SmartApps.
To install any of these, open the mobile app, select the Marketplace icon ([image]), and select the SmartApps tab.
SmartThings Recommends
Smart Lights by SmartThings
Control and automate Lights and Switches.
Safety & Security
Notify Me When by SmartThings
Receive notifications when anything happens in your home.
Enhanced Auto Lock Door by Arnaud
Automatically locks a specific door after X minutes when closed and unlocks it when open after X seconds.
Severe Weather Alert by SmartT…
See #3 under Safety and Security, Enhanced Autolock.