data: format sample json files

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.