Disable JSON compression

[pdinklag]

Some webservers integrate automatic compression of JSON files. This breaks asynchronous downloads of json.gz files via AJAX, which the MinecraftStats client then cannot decompress. The reasons are not entirely clear to me, but this has been a problem in the past.

In any event, since server-side compression can apparently be considered standard functionality, this means that MinecraftStats should no longer do it and move that responsibility to the admin.

[pdinklag]

Note to self: The transition may become a bit messy with many people going stuck in loading clocks. However, since something like #257 is currently missing, there is no nice way around that.

To avoid any confusion, the updater should actively delete any existing summary.json.gz file. This will force admins to do something, ideally ask what's up on Discord or just update the client on their own. The release notes should very clearly state this intent.