Refactor image resizing

[InfiniteStash]

Removes previous image resizing infrastructure in favor of JIT resizing via libvips. The upside is we can use thumbs of different sizes rather using 720p/1080p images for everything.

This requires CGO for linux, which breaks static linking on linux. It's proven very difficult to sort out on Windows, so the windows build will continue using the rather slow go resizing. As long as production builds use docker, it shouldn't cause any issues.

Overall it's pretty speedy, although for production I recommend using varnish or similar to cache.