AI: more AI tweaks

- Dangerous navigation areas are now much scarier to bots and aliens
- Marine bots can make barrels explode
- Marine bots yell for a medic sooner
- Marines follow the tech when he's hacking and officers, special
weapons experts, medics, and techs (in that order) otherwise.
- Spectating a squad leader will follow whoever is the leader of the
squad.
- Disabled code that assumed there were still 3 followers in each squad
and had O(n!) runtime.