Teach `data/fetch_from_gitlab.py` to save the JSON documents with
human-readable formatting and in sorted order.  Use `jq -S .` to format
our existing data the same way.  Formatted JSON documents are still
sufficient to test our crate's deserialization of them and will make
updates to new GitLab versions much more readable.

While at it, also teach the script to remove the user identities,
which we previously did by hand on each update.