There seems to be legal precedent for holding GitHub liable for distributing software that encourages piracy according to MGM v Grokster[1]. Unless I'm reading that incorrectly, if they can prove that the software "promotes" the infringement, the claim will stand.
No. The maintainers of Popcorn Time might be able to be held accountable, but GitHub is a platform. As long as GitHub follows the take-down process laid out by the DMCA, they have what is called "safe harbor", which means that they are immunized from claims for copyright infringement.
I think it's worth noting that's a US law and the way things are going with GDPR etc, it wouldn't surprise me if another country viewed things differently.
I'm not convinced the courts will see much of a difference between "company (Grokster) that encourages easy p2p piracy through its software" and "company (GitHub) that publicly hosts software that promotes easy p2p piracy".