Support for scratch-online inside comm.js

[marc-portier]

To make the extension work also in the online environment each launched instance of the communication-link (comm.js) should also provide this:

<?xml version="1.0"?>
<!DOCTYPE cross-domain-policy SYSTEM "http://www.adobe.com/xml/dtds/cross-domain-policy.dtd">
<cross-domain-policy>
    <allow-access-from domain="*.scratch.mit.edu" to-ports="*"/>
    <allow-access-from domain="*.media.mit.edu" to-ports="*"/>
</cross-domain-policy>

at URI /crossdomain.xml