(PDB-772) Modify the facts endpoint to return structured facts

This pull request causes the v4 facts endpoint to return typed and structured
facts while leaving the v3 responses stringified as before.  It also adds a
"name" column to the fact_paths table representing the first element of a
fact's path.  This column should be renamed to "fact" in the future to avoid
confusion with the many references to 'certname' as name.
I'm okay with this ordering as long as we don't make promises in the documentation about it. That way when we change it again, we aren't breaking our commitment. Not that I anticipate a change, just saying.