Add mod compatibility database caching

[mibac138]

Introduces caching for the mod compat data. The current HTTP API server doesn't send Last-Modified or ETag headers, however this PR has support for it, if the server were to send those headers at some point.

In case the API is unavailable (and when waiting for the server's response) the cached data is shown (if any is present).

The cache file is ~0.5MB which is small enough not to be an issue. If it ever became too big, we can implement on-disk compression to reduce the size.

The UI could probably be slightly improved to show a different message depending on the state: Loading..., or Updating... (cache present), but not sure if it's worth the effort for such a minor improvement.

Mostly supersedes #831 (except in the rare case when someone turns MP on for the first time, and the API is dead at the same time)

[rautamiekka]

Leaving the user in the dark is never acceptable. For all the user knows, something is broken, and you'll get complaints aplenty for that.

Likewise, implement the compression right now, not when it's needed and no one will update the mod.

[notfood]

I like this idea better. The user never needs the very latest compatibility database to get things working, it's fine if it's outdated a bit, better than none at the moment.