Keep GitHub issues readonly

Description

When this project migrated to Tor's GitLab, it's former issue tracker on GitHub was disabled. This move made all previous issues inaccessible.

Tasks

• Re-enable issue tracker on GitHub, so past issues can still be seem.
• Disable issue creation on GitHub. Done at !22 (merged).

Time estimation

• Complexity: negligible (0.1 day)
• Uncertainty: low (x1.1)
• Reference (adapted)