refactor levelForSort and prettyDate

Using new properties on participants for that (levelForSort and prettyDate) 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. 

The name 'column' for the sort property could be refactored for clarity (it's not always a column anymore, it's a sort function).

Note that I preserved prettyDate function indentation in first commit to ease commit review, the second commit is just reformat, it can safely be ignored.