godoc: don't select line numbers when highlighting source code

CSS3 introduces the user-select property, which you can use to prevent snippets
of text from being highlighted. This is also the technique used by Github to
avoid highlighting line numbers when highlighting source code.

For more information, see:

https://developer.mozilla.org/en-US/docs/Web/CSS/user-select
https://www.w3.org/TR/1999/WD-css3-userint-19990916#user-select

Fixes golang/go#18279

Change-Id: Ia5eb68d5a042d6ee2a1c8e7bbad753a35de5c487
Reviewed-on: https://go-review.googlesource.com/34278
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
2 files changed