Feature: Create User "Assignments" to data

[whatisgalen]

There are already permissions to control which data users have access to, but it would be cool for an admin user to create an "assignment" so that while a user has access to all data, they might be assigned a particular task to a subset of data. An "Assignment" would likely be its own model with the User as FK, and could have a json field to delimit which data were in the scope of the assignment.

This idea also reminds me of #11407 as there are probably other use cases for an expanded User "Profile" UI

[chiatt]

This seems like a good candidate for an Arches application rather than a modification to core.