Advanced dataset sharing / visibility extension

The base CKAN permissions and roles represent a simple use case that is very effective in many situations. There are many cases however where something more flexible / granular is desirable. This has led to a variety of extensions adding different functionality:
[ckanext-collaborators](https://github.com/okfn/ckanext-collaborators)
[ckanext-restricted](https://github.com/EnviDat/ckanext-restricted)
[ckanext-resourceauthorizer](https://github.com/etri-odp/ckanext-resourceauthorizer)
[ckanext-privatedatasets](https://github.com/conwetlab/ckanext-privatedatasets)

Some work at the resource level, others at the dataset. Some work by users, others by users and groups. Some only expand access while others include editing. I think an advanced permissions extension unifying all the functionality would be a good goal.

A potential implementation would be an access control list for each dataset with the options of:
admin - edit data, metadata and user roles
editor - edit data and metadata
viewer - view data and metadata
metadata only - view description and metadata - would need to request access to the data
hidden - dataset not visible / searchable by the users(s)
resource specific - access determined at the resource level using a similar set of options

The lists should allow entries for individual users or organizations, so one could quickly share a private dataset with a second organization in a few clicks. If a user and their organization are both in the list the user would take precedence. I have used resourceauthorizer for similar functionality, but the action needs to be repeated for each resource.

Start by comparing the four linked extensions—ckanext-collaborators, ckanext-restricted, ckanext-resourceauthorizer, and ckanext-privatedatasets—to understand their differing permission models. The issue provides no repository files or tests; done would require a decided, unified access-control design covering dataset, resource, user, and organization visibility and editing rules.

Written by the indexing model from the issue text.