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

Can't the three methods

- findNonExistingUserEmail
- findNonExistingUserName
- findNonExistingUserId

be only one using a switch ?