Jekyll contest buttons are hard-coded per MP, not generated

On course websites built with Jekyll, a per-assignment button on the assignments page (such as a contest or leaderboard link) is often a hard-coded Liquid block. The block loops over _data/assignments.yaml, matches the MP by name, and checks mp.visible. Shipping a new assignment adds no button by itself. Add the link constant in _data and copy the block. It stays hidden until that entry's visible flag is true, so check that the linked service is up before you flip the flag.