aqbanking: Move sources into a common sources.nix

The upstream URLs unfortunately aren't canonical, so let's have a
sources.nix which contains the release IDs used by the upstream sites to
associate a version of a particular package.

We're then going to create an updater script which should update the
sources.nix from there on.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @cillianderoiste, @urkud