Write a real "last_activity" to user information

The `GET /users` API call is returning, besides other properties, the `last_activity` property. Unfortunately, it is currently not of much use and in the future, other security and compliance tools need a better return value.

## Expected Behavior
The property should return when the last API call was executed.

## Current Behavior
Currently, the property returns only "interactive usage". This means, only the usage of the web interface through a session, but not when using credentials directly through HTTP authentication.