Fix failed Dask worker pod launch due to invalid username characters

[jmettes]

Since we've moved to using email addresses in Cognito, this code is now passing @ symbols into the Kubernetes worker pod and causing it to fail to create.

Failed to create pod in namespace sandbox - Pod "dask-worker-935744a16ab74944bff079717a6a5ba0-ls628" is invalid: metadata.labels: Invalid value: "jonathan.mettes@ga.gov.au": a valid label must be an empty string or consist of alphanumeric characters, '-', '' or '.', and must start and end with an alphanumeric character (e.g. 'MyValue', or 'my_value', or '12345', regex used for validation is '(([A-Za-z0-9][-A-Za-z0-9.]*)?[A-Za-z0-9])?')

Restrict jupyterhub_user parameter to alphanumeric characters.

[robbibt]

Awesome! I also hijacked this PR to add the code to automatically close any existing clusters

[emmaai]

tyty Jonathan!