Make a 2.0 release of LTI Authenticator

[yuvipanda]

LTI is an authentication mechanism commonly available in many LMSes - Canvas, Moodle, EdX, etc. JupyterHub has some support for it with LTIAuthenticator. It supports LTI 1.1, but 1.3 is newer and better supported.

Illumidesk does a lot of cool work around the LMS space, and @jgwerner from there has been working on modernising LTIAuthenticator and adding 1.3 support.

This task tracks reviewing and merging all that code in, as well as writing a blog post about if afterwards.

Outstanding PRs:

• Refactor LTI 1.1 validator jupyterhub/ltiauthenticator#44
• [WIP] LTI 1.3 Authenticator jupyterhub/ltiauthenticator#39