Support constraints for adding new content

[ericof]

Plone Classic allows the editor to restrict the addable content to a Folderish object, but this is currently not supported in Volto

Describe the solution you'd like

1. Volto add menu respects the constraints of a folderish
2. Volto add menu allows users to add/manage constraints

[pbauer]

Part 1 already works. Part 2 is still missing.

[davisagli]

Needs work in plone.restapi: https://github.com/plone/plone.restapi/issues/334