Accept CORS requests from dlang.org

[wilzbach]

Since a couple of days, examples can be run directly on dlang.org.
This uses Dpaste as a backend. However, it seems to me that

1. Dpaste doesn't use any caching
2. Any request to Dpaste takes at least two seconds

I am not sure how long it takes to start a Docker instance, but it might be worth checking whether the DTour is faster than DPaste

Also Dpaste has no way to open a snippet which is an often requested feature for the examples in dlang.org. So while I am not sure whether it's a good idea, but we we could have a /editor route that just loads the CodeMirror editor as we already have the backend ready to handle requests. Note that of course this js code could also be deployed under run.dlang.io`, so that for a normal user it won't even be detectable that it's powered by the backend of the Tour.