Prevent double posts

[wilzbach]

Maybe the GH API lags?

dlang/dmd#5855

[CyberShadow]

I think due to the way sharding is implemented, sometimes the request to create a resource go to one server, and an immediately following request to enumerate said resource goes to another which hasn't yet received the resource from the first server.

Now that we're adding persistence anyway, we can start transitioning to doing our own bookkeeping instead of asking GitHub every time for what we have to do (though being stateless has its advantages).