Add actions for enabling/disabling CAS on users

Problem/Motivation

It's hard to enable or disable CAS for a group of users in a single go.

Proposed resolution

Add an action for enabling or disabling CAS for users, for use with VBO or core's Bulk Operations.

Remaining tasks

Put together the necessary functionality.

User interface changes

An action would be available to be added to the Bulk Operations or VBO list for users on a user Views page.

API changes

TBD

Data model changes

n/a