References to Deleted Places Not Purged

[Enthalpy]

When Place A has a move option to Place B, and Place B is deleted, the command for A to move to B persists, causing a bug when the player tries to access Place A's move page in the player.

I support purging those references upon place deletion. Instead, we may simply have the player ignore those places. Bug demonstrated here.

[Arnaud Vié]

Player crash resolved by PR 23.

Further improvements on the editor (to purge references to a scene on deletion, or prevent deletion while references exist) could be useful, though.