Gametype strings backport

Backported from 2.2's code, originally created back in June 2017. Original description of the merge request from our internal repo:

This basically reorganises all code that gets gametypes by string or prints gametype strings out. This should change nothing in-game, though I intend to add Gametype_Names and G_GetGametypeByName to Lua before I consider this ready to go in. (Edit: eh doesn't really matter that much, I can add those later)