Add nonce also to legacy CSP

Pages that do not use the AppFramework have its CSP inherited from `\OC_Response::addSecurityHeaders`. While those are not many anymore, there are some examples such as the "Help" page.

To stay completely backwards-compatible we should also add the nonce to the legacy CSP response.

To test that open your browser console and open the help page. Without this you will get a JS error. With this you won't.

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>