Suggestion: Simplify checkout user procedure

I know, I know... I _wanted_ it to be complicated.

But since the user view proved to be an important tool, I am retracting my previous point of view.

Ideas:
- make the functionality less hidden in the settings
- keep the confirmation dialogs
- display the user name (will close #771)
- access via uuid is okay, but display uuid somewhere in contract view (backend task)
- allow to access a user via reference id (also backend task)