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

Yes, it is easier, and actually used in https://github.com/ezsystems/BehatBundle/pull/17/files#diff-6d6d3244da99f615507a8695749f8859R258
In theory, it is possible to have an already existing email for a different username, which could cause some complications, but in practice the chances of it happening are virtually impossible.