Adding relationships is no longer part of the generic save flow
anymore so move the add() below the parent::create() to make sure that
we create the user before we try to add a group to it.
Implement IdentityProvider_GroupDefinition - we expect that in Access.
Print exceptions, don't put them into the response body since they don't seem to show up otherwise.
Use type hinting in _set() and save some type-checking code.
Extract the model relationship code into its own separate function and
extend it so that we can fold in the relationships from modules that are
installed but not yet active.
Disable output buffering (recommended) and activate the gallery module
after install.