Control use of insecure content exceptions

Allows you to set whether users can add exceptions to allow mixed content for specific sites. This policy can be overridden for specific URL patterns using the 'InsecureContentAllowedForUrls' and 'InsecureContentBlockedForUrls' policies. If this policy is left not set, users will be allowed to add exceptions to allow blockable mixed content and disable autoupgrades for optionally blockable mixed content.