Flake8 webadmin

Since webadmin is not under heavy development at the moment, `flake8` its components should not impact the team workflow and reduce the `setup.cfg` configuration file.

/cc @aleksandra-tarkowska @ximenesuk

--no-rebase
Might this be better as:
```
self.fields['groups'].choices = [
    (group.id, group.name) for group in groups]
```