Add support for repo policy documents

Repositories have policy documents, in the case of f-droid.org there's inclusion policy and everything on the web also needs a privacy policy these days.

To make these available in F-Droid Client and also easier to access in the broader F-Droid eco system, they have to be made available in a repositories index file.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information