Move the jersey number input away from layers.json

Currently layers.json is otherwise adventure-agnostic, but it contains the "custom" category layer for Lätkäzombit's jersey number input.

This could be its own json file, and latkazombit.js should handle putting it somewhere nice - preferably not its own layer, so Konva would complain a little less about so many layers!

Read layers.json to identify the adventure-agnostic data and the custom category, then inspect latkazombit.js and its Konva setup. Move the jersey-number input data into a separate JSON file and have latkazombit.js place it without creating an unnecessary layer; verify that the Lätkäzombit input still appears correctly.

Written by the indexing model from the issue text.