Add the possibility to disable registration completely

[marceloschmidt]

Only admins can add/remove users to their infrastructure.

[engelgabriel]

Depends on #222

[graywolf336]

When adding users, will it be an invite style? As that's how I'm used to it but also just being able to add people without them confirming an invite would help for temporary users. Maybe even ability to add users and make them valid for one week only and expire their login until an admin/person with permissions extends their stay.

[marceloschmidt]

Duplicate of #195