GitHub App authentication support

[ViliusS]

This plugin is great for synching page content from GitHub repository. However, currently it requires a GitHub user account which then must be managed by someone. This can become a hassle if Grav website is owned by the company, not a person. Given that GitHub now requires 2FA this further complicates things.

It would be great if support for GitHub App authentication could be added. Not only this removes user-dependant authentication, but also gives larger GitHub API request rate limits and improves integration security.