R1:5b3a351852a7

Use pseudoelements, not Zero Width Space, to implement copy/paste behavior in Paste/Diffusion

Summary:
Depends on D19348. Ref T13105. When copying text from Paste or Diffusion, we'd like to copy only source, not line numbers.

We currently accomplish this with zero-width spaces plus a trigger that fires on "copy" in Paste and Diffusion. This is quite gross.

In the new-style Harbormaster logs, we use an approach that seems slightly better: CSS psuedoelements.

This isn't a complete solution (see also…