Standalone Overkill weapons
Currently, Overkill mutator changes a models of shotgun, machinegun and vortex and Overkill config changes their stats. But sometimes (for example, in Survival) we want to use Overkill weapons outside of Overkill mutator. So I've made them as completely standalone weapons that can be used in any gamemode or mutator.
Also, I've added blaster secondary to HMG and RPC.
This MR depends on !484 (merged).