Feature request - add CartONG 'Mapathon Dashbaord' to 'Explore' dropdown

[SColchester]

If the user follows 'Explore' --> 'Statistics' it takes them to the new HeiGIT stats page - which is perfect. However, the new stats page does not include a user leaderboard feature. This is a feature that was present in the old Missing Maps leaderboard. In order to fill this gap it would be great to also add a link in the 'Explore' dropdown to 'Mapathon Dashboard' http://mapathon.cartong.org/

Discussed in Missing Maps ops call on 3 October. Also checked with CartONG and Martin confirmed would be fine to add link to Missing Maps website.