R1:37f12a05ea62

Behold! Copy text from either side of a diff!

Summary:
Ref T12822. Ref T13161. By default, when users select text from a diff and copy it to the clipboard, they get both sides of the diff and all the line numbers. This is usually not what they intended to copy.

As of D20188, we use `content: attr(...)` to render line numbers. No browser copies this text, so that fixes line numbers.

We can use "user-select" CSS to visually prevent selection of line numbers and other stuff we don't want to copy. In…