Signature gauge for child scopes

Currently the number of signature required for an initiative is defined by the scope that it's attached to. The relation is one to one, an initiative has only one scope. We have a use case where we'd like to specify a number of signatures at different levels of scopes : - Parent - Child In order to compute the number of signatures for child scopes, we'd need to activate "Collect participant personal data on signature" and use an authorization handler that verifies that a user pertains to a scope to fill the form. https://github.com/OpenSourcePolitics/dev/issues/9