Set up git LFS

[pvutov]

Currently we're committing binary files like the terrain data into our repo. Instead we should put them in LFS. Care should be taken to not disrupt the workflow of non-technical users (github desktop claims first class LFS support, so it should be easy enough to make this change in a transparent way).

An issue is that we likely can't use github's free hosting service (see https://medium.com/@megastep/github-s-large-file-storage-is-no-panacea-for-open-source-quite-the-opposite-12c0e16a9a91). This means that the change will also require setting up our own LFS server.

Alternative solutions like dropbox can also be tried, but they must not complicate the setup for first-time users.