Prepare for 6.0 release with upgrade notice, disabling auto updates for 6.0, and code preparation to prevent conflicts between major versions

* New: option to install required free plugin from pro

* Improvement: no upgrade insecure requests network wide on per site activated sites

* Improvement: removing CSP rules

* Improvement: upgrade .htaccess http headers to advanced-headers.php

* Improvement: use manage_security capability

* Improvement: extend system status with versions of both free and premium

* Improvement: catch situation where pro is deactivated while free not active

* Fix: advanced-headers.php did not generated CSP correctly

* Fix: Browsers support for new report-to reporting attribute not good enought to use it yet.

Dropped PHP header option in favor of advanced-headers.php

* Improvement: New default Referrer Policy => strict-origin-when-cross-origin

* Improvement: New default for X-XSS header

* Improvement: Dropped Expect-CT header as it's now deprecated, integrated by default in certificates

* Improvement: Updated Permissions Policy with improved defaults, updated to new values as currently supported in browsers

* Improvement: dropped access-control-allow-origin header, as it can only be used dynamically, which is not feasible in most WP environments

* Improvement: new languages

* Improvement: several small bug fixes and typo's

Fix: roll back new license check 

* Improvement: language files

* Changed: license

Removed credentialless from CORP setting as it caused some issues with embedded videos

Fix: PHP <7 compatibility