Download restructuring

[Johannes Pfau via D.gnu]

I've now moved most downloads & integrated the 4.9.3 and 5.2.0 releases.
Next steps are:
- update travis symlinks
- delete old folder structure

There's one small problem: The Travis links are currently in
binaries/$HOST and these host folders should be removed at some point.

@Iain shall we move these symlinks to a new location? Maybe
ftp.gdcproject.org/ci/gdc-x or ftp.gdcproject.org/binaries/ci/gdc-x ?