Support GitHub repos that only allow checkout through web URLs

[dennisknothetelus]

Is your feature request related to a problem? Please describe.
My organization uses GitHub enterprise that requires SSO with our corporate IdP, which requires MFA. The organization has disabled repo access via SSH or Personal Access Tokens. Only checkout via web URLs (HTTPS) is allowed. I installed OpenGrok through the default container image, but was not able to configure OpenGrok to access such repositories.

Describe the solution you'd like
Extend the OpenGrok GitHub integration to allow access to repos that require checkout via web URLs. Provide detailed instructions how to install OpenGrok through a container image to access such repos.