Improvements to the latejoin job selection screen

[Lady Birb]

Allow people to join as AI only if there is a 'vacant' and ready core. Not only does it allow an AI to come in, it lets them come in if the core has been rebuilt elsewhere. Or multiple AI if multiple cores were built....

Yeah this is good.

1. Even if there is no roundstart AI, have a vacant core in the AI room for a latejoiner to jump into
   
2. Add the ability to fabricate Neural Net Processors that can be put into cyborgs or AI cores that latejoiners can enter.