sage: 6.8 -> 8.0

This "un-breaks" sage while also updating it to 8.0.

It compiles sage with its dependencies as one big pile, which is not
the best approach but definately better than nothing for now.

To be able to shrink the huge output pile a little, it also splits
docs from the rest of the output.

lib/maintainers.nix

@@ -627,6 +627,7 @@
   ThomasMader = "Thomas Mader <thomas.mader@gmail.com>";
   thoughtpolice = "Austin Seipp <aseipp@pobox.com>";
   timbertson = "Tim Cuthbertson <tim@gfxmonk.net>";
+  timokau = "Timo Kaufmann <timokau@zoho.com>";
   titanous = "Jonathan Rudenberg <jonathan@titanous.com>";
   tnias = "Philipp Bartsch <phil@grmr.de>";
   tohl = "Tomas Hlavaty <tom@logand.com>";