ContextSummary() was cutting the HTML which was added by nl2br because
it expected plain text elements as it's stripping and replacing text.
Instead this fix changes the behaviour to apply the nl2br after the text
changes have been made. That way we can't cut anything in the middle of
a HTML tag, but new lines, or paragraphs are replaced by BRs after,
should they exist.
- Added tests to ensure text is not cut in the middle of a sentence.
- Added test to ensure that <br>'s are added in the correct place should
the summary span between new lines.
