Async upload providers

[ginkoid]

At the moment, all upload providers are loaded synchronously with require(): https://github.com/redpwn/rctf/blob/163e25bc94f7083682399cc6057bca87f76bf57e/server/uploads/index.ts#L11-L13

We should use import() instead, such as in email providers: https://github.com/redpwn/rctf/blob/163e25bc94f7083682399cc6057bca87f76bf57e/server/email/index.ts#L20