gitit: init at 0.13.0.0

This was broken in `haskellPackages` for a long time, but it’s back!

Time to give it a place in the toplevel and a prope) treatment:

* Semi-static binary
* Remove all references to `Paths_` modules of dependencies
* gitit tries very hard to have a runtime dependency on GHC, disable
  that by default.

Also added myself as a maintainer, let’s try to keep this working from
now on.

Fixes https://github.com/NixOS/nixpkgs/issues/32377