Add GitHub auth

OpenHour is supposed to be easily installable into any personal or org repo.

For personal repos, the owner as well as all the collaborators should be able to track times and view reports.

For org repos, all org members should be able to track time, view their own times in reports, and org admins should be able to view reports aggregated across all members.

The issue defines separate access requirements for personal and organization repositories, including collaborator, member, and administrator permissions. Start by locating the existing authentication and permission entry points, then verify that each role can track time and view only the reports described. Done means GitHub authentication supports both repository types and the specified report access.

Written by the indexing model from the issue text.