web: cleanup CodeExcerpt / CodeExcerpt2 code split

Created by: slimsag

CodeExcerpt2 feels like a particularly bad name for something that is only enabled by developers in the case of:

// Dev flag for disabling syntax highlighting on search results pages.
const NO_SEARCH_HIGHLIGHTING = localStorage.getItem('noSearchHighlighting') !== null

This change renames it to CodeExcerptUnhighlighted and adds documentation to both which describes the difference between the two.

Helps #6992

Signed-off-by: Stephen Gutekanst [email protected]