Implement User Context / Object Management

Implement User management context/trait, Object Management, and Features file.

It is likely new functions/sentences may need to be added in the future, but AFAICT the most important should be done.

May depend on [ezpublish-kernel PR #1083](https://github.com/ezsystems/ezpublish-kernel/pull/1083) due to related recursive sudo issue.

Feedback welcome.

TODO:
 - [x] Further tests/sentences using id/email/password/content fields
 - [x] cleanup/refactor, use UserGroup Object Manager if needed/possible

@joaoinacio Just a question, shouldn't be easier to build the email using 
```
$email = $username . '@ez.no'; ?
```
Since the username will always be unique ?