Add public API to give developers the possibility to adjust the global CSP defaults

Allows to inject something into the default content policy. This is for
example useful when you're injecting Javascript code into a view belonging
to another controller and cannot modify its Content-Security-Policy itself.
Note that the adjustment is only applied to applications that use AppFramework
controllers.

To use this from your `app.php` use `\OC::$server->getContentSecurityPolicyManager()->addDefaultPolicy($policy)`,
$policy has to be of type `\OCP\AppFramework\Http\ContentSecurityPolicy`.

To test this add something like the following into an `app.php` of any enabled app:
```
$manager = \OC::$server->getContentSecurityPolicyManager();
$policy = new \OCP\AppFramework\Http\ContentSecurityPolicy(false);
$policy->addAllowedFrameDomain('asdf');
$policy->addAllowedScriptDomain('yolo.com');

$policy->allowInlineScript(false);
$manager->addDefaultPolicy($policy);
$policy = new \OCP\AppFramework\Http\ContentSecurityPolicy(false);
$policy->addAllowedFontDomain('yolo.com');
$manager->addDefaultPolicy($policy);

$policy = new \OCP\AppFramework\Http\ContentSecurityPolicy(false);
$policy->addAllowedFrameDomain('banana.com');
$manager->addDefaultPolicy($policy);
```

If you now open the files app the policy should be:

```
Content-Security-Policy:default-src 'none';script-src yolo.com 'self' 'unsafe-eval';style-src 'self' 'unsafe-inline';img-src 'self' data: blob:;font-src yolo.com 'self';connect-src 'self';media-src 'self';frame-src asdf banana.com 'self'
```