CORS: Use CORS proxy for all CORS-affected outbound requests

[adamziel]

Playground has a CORS proxy now. Today, it's used just for requesting git repositories to limit the potential for abuse. It would be highly useful to use it for all cross-origin requests, and perhaps reply with an error code if the target host or content type is not allowed. This is a pre-requisite to closing the long standing network access in the browser issue.

cc @brandonpayton

Related #1901

[adamziel]

Solved in #2076. I'm closing this for now, although I want to acknowledge we might need to backtrack and disable this feature if it's misused.