Elephant_stables -> elephant_stable.
- Renames the elephant_stables.xml templates to elephant_stable.xml (without plural s), as well as their generic template_*.xml parent, consistent with e.g the stable.
- Corrects all occurrences in simulation and map files accordingly.
- Renames the stable and elephant stable audio files.
- Drops pers_ from the barrack.png and stable.png icons.