Login with github as backend fails if base url contains additional path segment

**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.