Create purchased filter

When accessing stories by account id, if the 'purchased' filter is used, only stories that have been purchased will appear, and they will be sorted by how many times they have been purchased.
@kookster the semantics of the "highlighted" filter are likely to change in the future, though, so I don't think that a name change is in order. Right now, this is implemented using the portfolio, but I suspect that in the future it will be implemented with a combination of picks, portfolio, and other heuristics.