Making it play with the markdown preview JS

the edition body preview text was being covered-up
by the highlighting wrapper div. fixed that by making
the preview text sit outside the highlighter wrapper.
Match whole-words: so match key, not keywords
Using @rinogo's fork of the text highlighter JS

it contains a fix for scrollbar issues.

the plugin makes an assumption that scrollbar
width is fixed at 18. whereas, this fork uses
a calculated scrollbar width.

https://github.com/rinogo/jQuery-highlightTextarea

this problem surfaces when using a trackpad and
there are no scrollbars shown. however, there is
no styling that tells the plugin that there are
no scrollbars shown, so it uses it's default 18px
padding.
Re-formatting to remove indentation mismatch

also, improves readability.
Merge pull request #1364 from alphagov/words-to-avoid-highlighter

Fixing a few issues found on preview