Disallow form-action in CSP to limit outgoing connections

See https://github.com/PrivateBin/PrivateBin/issues/272

cfg/conf.sample.php

@@ -73,7 +73,7 @@ languageselection = false
 ; scripts or run your site behind certain DDoS-protection services.
 ; Check the documentation at https://content-security-policy.com/
 ; Note: If you use a bootstrap theme, you can remove the allow-popups from the sandbox restrictions.
-; cspheader = "default-src 'none'; manifest-src 'self'; connect-src *; script-src 'self'; style-src 'self'; font-src 'self'; img-src 'self' data:; referrer no-referrer; sandbox allow-same-origin allow-scripts allow-forms allow-popups"
+; cspheader = "default-src 'none'; manifest-src 'self'; connect-src *; form-action 'none'; script-src 'self'; style-src 'self'; font-src 'self'; img-src 'self' data:; referrer no-referrer; sandbox allow-same-origin allow-scripts allow-forms allow-popups"
 
 ; stay compatible with PrivateBin Alpha 0.19, less secure
 ; if enabled will use base64.js version 1.7 instead of 2.1.9 and sha1 instead of

lib/Configuration.php

@@ -54,7 +54,7 @@ class Configuration
             'urlshortener'             => '',
             'qrcode'                   => true,
             'icon'                     => 'identicon',
-            'cspheader'                => 'default-src \'none\'; manifest-src \'self\'; connect-src *; script-src \'self\'; style-src \'self\'; font-src \'self\'; img-src \'self\' data:; referrer no-referrer; sandbox allow-same-origin allow-scripts allow-forms allow-popups',
+            'cspheader'                => 'default-src \'none\'; manifest-src \'self\'; connect-src *; form-action \'none\'; script-src \'self\'; style-src \'self\'; font-src \'self\'; img-src \'self\' data:; referrer no-referrer; sandbox allow-same-origin allow-scripts allow-forms allow-popups',
             'zerobincompatibility'     => false,
         ),
         'expire' => array(