Throw deprecation warning if directive contains 'none' and other values

Problem/Motivation

To prepare for changing the behaviour of handing 'none' in a directive's values in CSP 2.0, reduceSourceList() should throw a deprecation warning if a directive contains 'none' alongside other sources (except valid keywords like 'report-sample')

Steps to reproduce

Proposed resolution

If a directive contains 'none' alongside other sources (domains, protocols, 'unsafe-inline', etc), throw a deprecation warning that a site should remove any undesired sources.

Remaining tasks

Create change record with info on upcoming change in 2.0, and how to address warning in 8.x-1.x.

User interface changes

API changes

Data model changes