Auth0 backend

**Is your feature request related to a problem? Please describe.**
Auth0 is a popular service that centralizes authentication needs. As a developer, I like to use tools that I already know.

**Describe the solution you'd like**
`git-gateway` supports third-party services that handle JWT tokens. Integrating Auth0 as an official backend in Netlify CMS could raise its adoption.

**Describe alternatives you've considered**
I considered using GitLab – where my repositories live – and Netlify Identity but both are not as flexible as Auth0 in case of authentication.

**Additional context**
Auth0 offers a great API which could be easily integrated. I would do it, but don't know where to start.