Add a new unit to the game about 0ad

[luumu]

New unit: Celtic knife-wielding skeleton
Abilities: Quick (200% walk speed), high defense, low attack, aura gives -10% accuracy for enemy infantry. Implemented for the Britons, though also created template_unit_champion_skeleton and template_unit_champion_infantry_skeleton, they start with one and the unit is buildable at the city phase by the city center building.
Sound effects: Added sounds for, move, select, attack, garrison, and death of the skeleton. The sound effects were taken from freesound.org
cracking_knuckles: https://freesound.org/people/AnusProductions/sounds/402759/
skeleton_walk: https://freesound.org/people/AntumDeluge/sounds/188034/
kneeling_bones: https://freesound.org/people/Kneeling/sounds/473526/
rattling_bones: https://freesound.org/people/spookymodem/sounds/202102/
falling_bones: https://freesound.org/people/spookymodem/sounds/202091/
All of the sounds are licensed under the Creative Commons 0 License (public domain)
Screenshots: