Partially decouple disease resist from enviro protection

[anoobindisguise]

Summary

Balance "gas masks no longer make you immune to getting sick"

Purpose of change

Total immunity to disease from wearing gas masks was silly and made having bad health a non issue, and made disease basically a complete non issue. It also didn't make sense - the mouth isn't the only vector for disease, and there's no way to wear a gas mask 24/7 because you have to take it off to eat, drink, etc, which exposes you to disease in intervals.

Describe the solution

Mouth environment protection matters, but much less so. You are ( (mouth environment protection)^0.3 / 2 )+ 1 times less likely to get sick. If you have 1 mouth protection, this is 33% fewer diseases. If you have 15 protection (24/7 gas mask or the filter bionic) you are about less than half as likely to get sick. If you are dead-set on being immune to diseases, and live in a plastic bubble (powered phase immersion suit) disease rate is cut down to almost 1/3.

Describe alternatives you've considered

Different rates.

Testing

This will be a little tough to test due to the time required.

Additional context

Better hunt down those panaecii!

[Inglonias]

There's a main menu tip that suggests that gas masks will keep you from getting sick. Suggest you edit that tip if you plan to merge this.

Relevant tip is here: https://github.com/CleverRaven/Cataclysm-DDA/blob/master/data/core/tips.json#L640

[Fris0uman]

I think it'd be better to fix the randomly catching cold out of nowhere mechanic first.

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