Allow capturing brief summary of puzzles

[ebroder]

For two years in a row, Mystery Hunt has included a brief description of a puzzle, available before the puzzle unlocks. Having an option to present that summary in Jolly Roger (especially on the puzzle list page) could be a useful way of helping people decide what puzzles they want to work on. Having a flow for making it editable by anyone could be a good hedge if teams drop the feature in the future (or leave it out, as Cardinality did for the outer dimensions)

We probably want some limitation on length here, just so that the list page doesn't get out of control, along with UI for showing and hiding them.