R1:5f0d8e09d888

Use "user-select: none" to provide a visual cue about copy/paste JS magic

Summary:
- For line numbers, use "user-select: none" to make them unselectable. This provides a stronger visual cue that copy/paste is enchanted.
- In Paste, make it look sensible again after the blame-on-blame refactor in Diffusion. See also TODO to share this code formally.
- In Diffusion, use the "phabricator-oncopy" behavior.

NOTE: I left blame/commit columns selectable in Diffusion, since you might reasonably want to…