Integrate R2 (or similar S3 apis)

[FireMario211]

Adds a new column: managed_download_link, which is used to redirect downloads and is prioritized over download_link. Once a mod is approved, the index asynchronously re-uploads the .geode file to the S3 storage, if it is enabled, and updates managed_download_link. This lets us serve downloads from the fast and reliable Cloudflare R2 storage and not worry about mods becoming undownloadable if someone messes up their repository or such.

There is a migration that runs every time the server is started and uploads mods to S3 that do not have the managed link set. The criteria for a specific mod version to be auto migrated to S3 upon startup are:

• The mod version is approved
• The mod version targets a GD version we currently support (currently 2.2081 and 2.2082)
• The mod version is the latest version of the given mod to support its GD version. This could mean for example, if we choose to support 2.2074 and 2.2081, a mod could have up to 2 versions auto migrated: the latest one to target 2.2074 and the latest to target 2.2081.

This PR also has some unrelated refactors, such as small idiomatic Rust improvements and the use of a single reqwest::Client in the entire index instead of creating a new client per http request.

[Fleeym]

By the way, wait for #78 to merge, it simplifies storage.rs and you can implement a backend for it.

[Fleeym]

Forgot to mention that it's merged now.