Integrate save-to-library login flow

Here is an example link to a PDP that will request that a user logs in if they have not logged in, and then will save the target paper to their library: https://www.semanticscholar.org/paper/Large-Pose-3D-Face-Reconstruction-from-a-Single-via-Jackson-Bulat/066d7e4adf6ef40ddd1eaf81efc93f35c4f462e2?save_to_library=true. Note the `?save_to_library=true` query parameter.