remove GitHub username from changelog
remove GitHub username from changelog

This modifies the changelog such that the configured username isn't
used. Only the user login is used.

If the login is `maxm` and the configured username is `Max Mustermann`,
the changelog entry would have contained:

```
(`Max Mustermann  <https://github.com/maxm>`_)
```

And with this change the entry would now contain:

```
(`maxm  <https://github.com/maxm>`_)
```

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