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.
account_id usually comes from the route - this controller is the one used by the /api/v1/accounts/212/stories endpoint. We're just requiring that the `:account_id` param be present for the "highlighted" filter to have any effect because for now "highlighted" just refers to an account's portfolio and without an account it is meaningless.