Add more granular contributor statistics

As the manager of pt-pt I want to promote to admins the users which have more merit.

Currently I'm using the "Top contributors" list to control who are the top contributors for the translation, however, as the list grows this list will have limitations and we should have a method of selecting admins by their merit.

I suggest that on the member list for the language (or on a report tab on that screen accessible by managers and admins), there should be a report with the following columns (for the language):

- Total approved translations;
- Total translations submitted;
- Total rejected translations;
- Date of last submitted translation;
- Date of joining the site ("Member for").