Support for non-standard download directories

It would be nice, if Keybase would support non-standard download directories for files shared in chat. On my setup the `Downloads` directory is missing so Keybase just created one and put the downloaded file there.

I see that [this is the place](https://github.com/keybase/client/blob/49653314fe2240134cf90c6ae86efa5eb6f7f1fe/shared/util/file.desktop.js#L32) that seems to control this right now. Also looks like at least on debian (and thus ubuntu) there's a package called `xdg-user-dirs` that manages the location of these "special" directories. So indeed my downloads directory is specified in `$HOME/.config/user-dirs.dirs`.

Runnig on Keybase GUI Version: 1.0.40-20180123171551+6ea38e5a0

Start in shared/util/file.desktop.js around line 32 and inspect how shared chat files choose their download directory. Compare that behavior with the xdg-user-dirs configuration in $HOME/.config/user-dirs.dirs, then verify that downloads use the configured location instead of creating a default Downloads directory.

Written by the indexing model from the issue text.