Thumbnail badges (#191)

This adds a new type of badges, Thumbnail badges (see #191).

As in #270, I have split this from the `feature-191-thumbnail-badges_and_95-hierarchical-badges+` branch, which itself is a rebase of `feature-thumbnail_badges_#191_hierarchical_badge_#95`. I'm keeping both branches around in order to resolve potential issues due to rebase and split, until this and #270 is merged.

Yet missing - can also be implemented after merging.

The thumbnail images should use "real" image files instead of of inline data in order to save bandwidth and be IE<8 compatible.