Provide username_safe label for kubespawners modern escaping

[consideRatio]

Mapping an NFS server directory name like my-2ename-40example-2ecom to the username [email protected] can be done using the jupyterhub-groups-exporter provided labels currently, specifically the username_escaped label. But KubeSpawner now has two escaping schemes, "escape" and "safe". "safe" is the new default, while "escape" has been used for a long time, so it is useful to be able to map escaped usernames from either scheme.

With this PR, we ensure that we can map directory names to usernames, regardless of whether the directory names were created from escaped usernames under the old or new escaping scheme. This has been verified in jupyterhub/grafana-dashboards#168.

actual username:          [email protected]
legacy scheme ("escape"): my-2ename-40example-2ecom
modern scheme ("safe"):   my-name-example-com---abcd1234