Mitigation rework

[DaloLorn]

Per the Discord to-do list:

• Make shield mitigation more like it is in Sins of a Solar Empire.
• Make shield hardeners accelerate mitigation ramp-up, delay mitigation decay, and reduce or increase incoming damage based on its size (so that damage beneath a given threshold is amplified instead of being reduced)

In Sins, shield mitigation starts at 15%, grows by +1% for every 10 damage taken (whether this is measured before or after being reduced by mitigation is not confirmed, but the growth itself occurs after the damage is applied), and decays at a rate of -1.25% per second.

However, while this growth rate is viable in Sins, where the amount of SP and damage in the game remains fairly constant, it seems inappropriate for SR2. Consequently, I think I would prefer to map it to +1% for every X% of the ship's maximum SP in pre-mitigated damage. How much, exactly, is something I'm not sure of yet. Possibly 1% for every 0.4% of the ship's shield capacity, so you cap out at around 50-70% SP depending on shield type.

Hardeners could then increase the ramp-up rate and slow down decay, OR raise minimum mitigation, OR modify all incoming damage to approach a certain damage threshold. Possibly some combination of the three... or they could each be implemented by different subsystems.