refactor levelForSort and prettyDate
using new properties on participants for that kind of breaks
databinding, if you update the time property you have to think that you
need to update prettyDate property and levelForSort too.
Therefore I implemented prettyDate through an angular filter, and
levelForSort through a sort function. Then name 'column' for the sort
property could be refactored for clarity.
Note that I preserved prettyDate function indentation to ease commit
review, I'll review indentation in a separate commit.
reformat prettydate function (no change besides indentation)