[Long-term] Optimize the autopush cycle

[3vorp]

The vast majority of the current autopush code, while functional, is really poorly optimized. Almost all requests are blocking, meaning the previous request stops all subsequent requests until it resolves, which is really bad design.

Having looked at the codebase, I've identified a few poorly optimized blocking loops:

• Result/council messages are posted one by one.
• Versions are downloaded one by one.
• Textures are downloaded one by one.
• Contributions are posted pack by pack.
• Comparison images have their parts loaded one by one.
• GitHub pushing is done in a triple-nested for loop completely synchronously.
• Packs in cron jobs are iterated over synchronously

I'll probably implement these gradually which is why I've formatted this issue as a checklist and as a long-term goal (I'm not sure all of them are necessary and I don't want to cause unnecessary difficulty if I don't need to).