Convert maintainer file entries to attributes, add github handles

Based on https://github.com/NixOS/nixpkgs/pull/34842, the
nix-instantiate output was pretty-printed and the validity of the github handles
manually verified, by automatically checking whether the user handles exist on
github (https://github.com/userhandle, status 200 or 404).
Each handle under 5 characters was manually checked (because the collision
probability with non-maintainer accounts is high), each missing entry was
manually researched.

The script used is kept in `maintainers/scripts` as an example of how to work
with the mainainers list through nix’ JSON interface.

lib/default.nix

@@ -21,7 +21,7 @@ let
 
     # packaging
     customisation = callLibs ./customisation.nix;
-    maintainers = callLibs ./maintainers.nix;
+    maintainers = import ./maintainers.nix;
     meta = callLibs ./meta.nix;
     sources = callLibs ./sources.nix;