Define and show badges for achievements

[joergreichert]

Current state: All users are equal.

Motivation: Getting badges as trophies for your achievments ( #24 ) or current leader board position ( #60 ) can be motivating.

Future state: Badges are shown in user profile sidebar, when certain achivements has been reached by the user. We should come up some cool icons representing those badges (either selecting existing ones or design own). Additionally those badges might be shown within the leader list ( #60 ), in the map ( #58 ) or the watering entries.