Add and remove custom fields

The form (for adding a custom field) is a bit rough, but I suspect we will use plexus-forms for that later.

To ease generalization of functionality over our different object types (pedigree, nest, member), the FocusLevel constant is renamed to ObjectType and used to parameterize by object type. Similarly, the Focus record is generalized to Ref as it is useful for more than storing the focussed object.