De-duplicate diff lines from backend

Why do we still have distinct parallelDiffLines and highlightedDiffLines objects coming from the backend? The backend shouldn't need to duplicate this data for us when we can just normalize the "inline" and "side-by-side" diff views on our own in the frontend/ 🤔

in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21063#note_94254197

Problem we're solving

Backend is sending the same information twice, structured differently, when we can handle that distinction in the frontend.

/cc @smcgivern @timzallmann