Invite to create new resources

[joepio]

The #23 Invites model allows for granting some Agent read or write rights to an existing resource. But what if you want to invite people to create something new? One usecase for this is surveys: #32

If we want to create a new thing, we need to know:

• What the target parent is to be
• What the resource class is
• Whether some fields are pre-filled (default values)
• Whether the questions shown in the form have custom labels / fields?

But we could also say: just invite people to a parent that contains this information (such as the class). A possible approach to this is constrained collections: #37