HTTP\HTTPS incompatibility warning

[tcibinan]

Due to the limitations of HTTP usage in modern browsers (HTTPS prevails) calls to HTTP protocol from HTTPS domains is forbidden. In terms of data2graph it means that f.e. GitHub Pages deployment of data2graph cannot be used with HTTP graph urls. Only HTTPS graph links are allowed.

Nevertheless calls from HTTP domain to another HTTP domain is allowed. It means that self-hosted version of data2graph is capable of displaying graphs by HTTP urls.

As long as it is pretty common issue for data2graph deployments it should be managed properly and warning should be shown to a user.

[tcibinan]

Also information on the cors rules configuration should be added to the warning.