Message detail highlighting

[michael-caktus]

Creates a second Message Detail serializer, MessageHighlightsSerializer, which takes the same fields as the regular one but instead of subject and body, returns highlighted_subject and highlighted_body.
Does so by getting query param "highlights" provided by front end. If highlights are present, the view will use the MessageHighlightsSerializer. The param is a comma separated list of values to be highlighted. The serializer uses an internal _highlight method to do the highlighting.
The highlighter uses some regex to capture a case-insensitive equivalent of the highlight provided, wrap it with "" tags and then stick it back in where it was before. I'm not sure how well regex will capture special characters, å ø π, so on. I don't know if that's a concern. I suspect people won't be searching much on those characters.