Switch to using a normal <a href="..."> link for search result
clickthrough. Apart from generally giving a better experience, this means that
it also works on html messages. The problem there was that we were attaching
onClick handlers to <span>s which we were then flattening into HTML with
ReactDOMServer (which meant the onClick handlers were never attached to React's
list of listeners).
To make this work without jumping through React hoops, the highlighter now
returns either a list of strings or a list of nodes, depending on whether we
are dealing with an HTML event or a text one. We therefore have a separate
HtmlHighlighter and TextHighlighter.
Factor out a Highlighter class to avoid passing round the static state
everywhere.
Add an optional 'opts' argument which can take an 'onHighlightClick' member.
