Disease Code Partial Rework

## Changes:
* Cuts /disease variables from 28 to 23 (includes new ones)
* Quite a few variables that survived the cut have been renamed
* A disease may now have more than one spread type (spread_type --> spread_flags)
* Added "infectivity" variable which replaces a prob(65), defaults to 65
* The affected_species variable is now viable_mobtypes, and lists typepaths
* Adds defines for a bunch of stuff
* Changes some fucking weird /const variable on /disease into a define
* Adds some new procs to try to reduce shit

This adds nothing cool for players, but paves the way for something I want to do and should improve the code.

<b> Does not touch Giacom's Advanced diseases, besides updating necessary vars </b>

These spread flags are already defined in code/datums/disease.dm