Extend GitHub Oauth2 strategy for use with GitHub Enterprise

[jabellard]

Description

The GitHub Oauth2 SSO strategy is built on top of the passport-github2 package. That package, by default, is configured to use public GitHub as the identity provider:
Authorization URL Setting
Token URL Setting
User Profile URL Setting
User Email URL Setting

Those defaults can be overridden so that an instance of GitHub Enterprise can be used as the identity provider.
The purpose of this PR is to enable that by setting the following environment variables:

• GITHUB_AUTHORIZATION_URL: Authorization endpoint.
• GITHUB_TOKEN_URL: Endpoint to retrieve access token.
• GITHUB_USER_PROFILE_URL: Endpoint to retrieve general user info (e.g., full name).
• GITHUB_USER_EMAIL_URL: Endpoint to retrieve user emails.

Sample configuration:

# New configuration settings================================================
GITHUB_AUTHORIZATION_URL=https://{{ENTERPRISE_INSTANCE_HOST_NAME}}/login/oauth/authorize
GITHUB_TOKEN_URL=https://{{ENTERPRISE_INSTANCE_HOST_NAME}}/login/oauth/access_token
GITHUB_USER_PROFILE_URL=https:/{{ENTERPRISE_INSTANCE_HOST_NAME}}/api/v3/user
GITHUB_USER_EMAIL_URL=https://{{ENTERPRISE_INSTANCE_HOST_NAME}}/api/v3/user/emails

# Configuration settings that remain unchanged in meaning============================= 
GITHUB_CLIENT_ID=*****************************************
GITHUB_CLIENT_SECRET=*****************************************
GITHUB_CALLBACK_URL=http://localhost:3170/v1/auth/github/callback
GITHUB_SCOPE="user:email"

The changes introduced in this PR are completed backwards compatible. As such, if if the intent is to use public GitHub as the identity provider, nothing changes. However, to use an instance of GitHub Enterprise as the identity provider, the environment variables outlined above must be set appropriately.

Checks

• My pull request adheres to the code style of this project
• All the tests have passed

[r-brown]

Any progress here? May we expect this PR merged anytime soon?

[alastair-drong-wd]

Any progress here? May we expect this PR merged anytime soon?

See here, @r-brown, but the tl;dr is that they're rolling out an overlapping feature for Enterprise users, so they don't want it in the community version.