Restructuring to support multiple spacecraft and spacepeople

This probably introduces bugs since we don't have 100% test coverage and it adds an extra object between users and spacecraft and fundamentally changes the structure, but there's also a chance it works fine.

Spacepeople are almost entirely implemented, all that is needed is for spacecraft to have a method to create them and to have it periodically run via rake tasks.

rake db:seed must now be run on each instance of issflyby once to obtain the default ISS row in spacecrafts.