Made the user registration content type configurable

[bdunogier]

Question	Answer
Tickets	EZP-XXXXX
Bug fix?	no
New feature?	yes
BC breaks?	no
Tests pass?	yes/no
Doc needed?	yes
License	GPL-2.0

Makes the user content type used for registration configurable, like the destination group.

ibexa:
  system:
    site:
      user_registration:

        # Content type identifier used for registration.
        user_type_identifier: "customer"

        # Content id of the user group where users who register are created.
        group_id:             11

Checklist:

• Implement tests
• Coding standards ($ composer fix-cs)

[ViniTou]

Any reason why ConfigurableSudoRepositoryLoader was dropped here?

[bdunogier]

I was gonna say "because it doesn't need it anymore", but actually it seems that it does...