Load content from any GitHub repo path

[chalin]

This requests for an alternative solution to the problem reported in #637. Namely, that it be possible to load DartPad content from any given GitHub repo path, such as https://github.com/dart-lang/site-www/tree/master/examples/language-tour/args/bin.

This example DartPad would be access via ?id=dart-lang/site-www/tree/master/examples/language-tour/args/bin.

Or more generally in the GitHub REST API: ?id=:owner/:repo/:path

cc @kwalrath @Sfshaza

[kwalrath]

cc @RedBrogdon

[RedBrogdon]

Added #1096 to track the ability to load individual exercises from a repo that houses a collection of them. We might still want to do this, though, so I'm leaving it open.

[parlough]

This was supported in a limited capacity but the newest frontend doesn't support it, due to the more focused scope. I'll close this in favor of #2733 and #2871 which may be alternatives worth exploring.

If anyone would still like to see this implemented, please open a new issue. Thanks so much!