Fixing a few issues found on preview

1. we allow editors to preview the markdown by making
an ajax call and rendering the response on a div that
replaces the textarea.
the edition body preview div was being covered-up
by the highlighting wrapper div. fixed that by making
the preview text sit outside the highlighter wrapper.

2. also piggybacking an enhancement to match whole
words only, so match 'key' and not 'keywords'.

3. we've switched to using a different fork of the highlighter
JS widget so that we can make highlighting work with long
documents. the old JS was trying to accommodate for a
scrollbar when there wasn't one, because we're using a
trackpad. this was the one @tekin mentioned.