Adds separate LevelUpPalette to GainsExperience

Needed for TS mod, as TS' own veteran crate effect anim uses a different palette than the rank chevrons. Would allow us to remove the RA backfall.
As this is a very small & simple change and didn't show any regressions in the shipping mods during testing, I hope this won't fall under the feature freeze.

Prerequisite for a larger TS mod update I have in the pipeline.