LTI JupyterHub Authenticator does not properly validate JWT Signature
### Impact
Only users that has configured a JupyterHub installation to use the authenticator class `LTI13Authenticator` are influenced.
LTI13Authenticator that was introduced in `jupyterhub-ltiauthenticator` 1.3.0 wasn't validating JWT signatures. This is believed to allow the LTI13Authenticator to authorize a forged request granting access to existing and new user identities.
### Patches
None.
### Workarounds
None.
### References
- This code segment
https://github.com/jupyterhub/ltiauthenticator/blob/3feec2e81b9d3b0ad6b58ab4226af640833039f3/ltiauthenticator/lti13/validator.py#L122-L164 didn't validate a JWT signature.