Assign a new registered user to a group

Followup of #1621 (closed)
Refs https://redmine.teklia.com/issues/4968

When a new guest account registers on an instance, we may want to provide them with some shared ponos farms/agents.

To do that, we need to assign them automatically to a group.

1. create an optional configurations setting signup_default_group which optionaly points towards an existing group ID
2. when the user hits successfully the VerifyEmail endpoint, add them to this group (when it's set)