Preliminary secondary antag system

A single ling can now appear in any game mode as a sole secondary antagonist in addition to the normal antags. The odds of this happening are:

75% chance in traitor and double agents
50% chance in cult, meteor, nuke, revolution, and wizard
25% chance in blob and malf
No chance in lings/Tratorling (since they're already there)

This was intended as a replacement for the ling game mode (and perhaps the traitor+ling game mode as well) as a large group of players have suggested that the defensively oriented newlings cannot or will not effectively terrorize a crew enough to carry a round by themselves without help. The removal of ling mode doesn't have to be done for this update to work, but is merely a suggestion. At the very least the frequency of ling only rounds can be reduced.

Lings that are inserted into roundtypes that don't allow for proper escape will get survival objectives as to not render their greentext impossible.

Secondary antag assignment functions independent of main antag assignment so traitorlings, wizlings, nukelings (just one of them), and revheadlings are all rare possibilities. This respects protected/restricted jobs so there's no chance of buggy Malflings or borglings.

The code has been set up to allow for multiple secondary antags to be added, but no modes will spawn more than one secondary antag right now as a precaution against poor balancing. Likewise it would not be hard to allow different kinds of antags in the secondary antag system, but this has not been added to this commit, again as a precaution on balance.