This adds thumbnails to the gallery (#26). Mandalas are not currently animated (I'm worried that doing this for lots of mandalas would severely impact performance). It also adds a loading throbber for the gallery page.
If a gallery item can't be deserialized, a sad face is shown in place of a thumbnail, with a more detailed explanation on mouseover.
Currently the implementation is not as clean as I'd like. I'll clean it up in a separate PR, as it will involve factoring out logic from the creature and mandala pages (similar in concept to #189 / #190), which would add lots of noise to this diff.
Atul Varma