R1:4e91ad276d8a

Prevent copying Harbormaster build log line numbers with CSS psuedocontent instead of ZWS

Summary:
Depends on D19165. Ref T13088. Currently, in other applications, we use Zero Width Spaces and Javascript "copy" listeners to prevent line numbers from being copied. This isn't terribly elegant.

Modern browsers support a second approach: using psuedo-elements with `content`. Try this in Harbormaster since it's conceptually cleaner, at least. One immediate drawback is that Command-F can't find this text…