Add duplicate detection to announcebuild

[FloatingMilkshake]

Prevents announcing an Insider build with /announcebuild if it has already been announced before. Can be overridden with the force_reannounce option. This acts as collision detection too.

PRing because this feels like a suspiciously simple change—did I miss anything important?

[Erisa]

This seems fine.

There could be the potential, depending how close together the commands are, that the delay having to pull from / push to Redis will be big enough that the conflict is allowed anyway. if you wanted to reduce those odds, you could store the latest build in memory as well.
That was something I discovered when doing the warning collision detection: the more time you spend on I/O before being able to check, the greater the window where there can be a conflict anyway.

[FloatingMilkshake]

That's fair. I tried to send /announcebuild twice rapidly when testing, and found that my second command was refused before the first one was even responded to—so it seems to work pretty quickly. But once the set fills up a little, maybe it'll become an issue? I think I will probably leave it how it is for now, and if it becomes an issue later I'll consider storing the latest build in memory