Added Gunnery School of Nuln + Hired Swords overhaul

Added new warband Gunnery School of Nuln
Full review of the Characters catalog to fix existing mistakes and skills/equipment
Introduced new system for Warband rating calculation (most of the existing warbands still need to be updated to use it)
Removed a wrong constraint in the Lizardman catalog (Skink Braves should not be limited to only 2 models)

Full changelog: v0.2.4...v0.3.0