Describe the bug
If you use github as backend the following basic setup is required:

• run your own github auth server backend (see https://github.com/vencax/netlify-cms-github-oauth-provider) - https://mygithub-auth-backend.org as an example
  -create oauth app in github and configure callback url pointing to your github auth server backend (https://mygithub-auth-backend.org/callback)
• set base_url in config.yml according to your github auth server backend (https://mygithub-auth-backend.org)
  --> This works well if the base url does not contain any path segment. This does not work if you use an additional path segment in your github auth server.

Example:

• callback url -> https://mygithub-auth-backend.org/oauth2/callback
• base_url in config.yml: https://mygithub-auth-backend.org/oauth2

The authentication and token creation with github is successfull but the communication between the auth popup and the original page fails.

To Reproduce

• setup github auth backend with additional path segment
• adjust base_url in config
• try to login

Expected behavior
Login should work

Applicable Versions:

• Netlify CMS version: app - 2.12.9, core: 2.24.4, cms: 2.10.45
• Git provider: Github
• OS: windows 10
• Browser version Chrome 80

Additional context
It seems that the following check in netlify-auth.js is not working:
e.origin !== this.base_url
I think origin is always just the host without any path segment.