Query last submissions

[ogoletti]

From the admin/COURSEID/submissionspage, the query form should allow to filter only last submissions.

While waiting for #613 to be fixed, another easy work around is to replay submissions from thes submissionspage. In order to fix a switch from last to best, there is no choice, one need to rerun all submissions for the task. The query from the page is straightforward. But in order to fix a switch from best to last, we could just rerun the last submission of each student. Sadly, this filter is not available in the query form.