Document Type permissions per user group

Questions
1

Right now you can update node permissions for specific user groups in the backoffice.

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What I need is permissions for specific document types.

Let’s assume I have this tree:

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  1. I don’t want to allow editors to create a new document type Layout within the page. This page should not be re-used.

  2. The same goes for several other pages where I want specific permissions.
    I do have a few document types which act as folders. I want users to be able to add nodes to these folders, but not change the folders themselves.

  3. All public pages are composed by modules which are stored in Nested Content items, therefore they are document types too (IPublishedElement). I do want to restrict CRUD access to these elements too per user group.

Any ideas how to solve this issue?

I do not want a an angular-only solution/tweak. This has to be a security feature and therefore also implemented on the server-side.

PS. I have already started modifying the Umbraco source code and adding this myself, but this has proofed to be too complex for me yet. Thread here: https://twitter.com/tobithefox/status/1109089968239099904

This is a companion discussion topic for the original entry at https://our.umbraco.com/forum/98585-document-type-permissions-per-user-group