Epic: #1707

In #1764 we made it so that all existing resources have an "unknown" space. Let's say now that resources can have multiple spaces. What does this mean for each of our core resources? What would this mean for popular resource types? Reach out to authors if necessary.

This story should have answers for the following questions:

• What impact does this have on the use cases defined by Come up with a breadth of pipeline use cases to explore how spaces may be used #1766? What changes would be expected? How is the initial UX for someone picking up Concourse affected?
• What does it mean for a job to not define spaces? Do we have a default space?