One annoying thing about the `set-siteconfig` management command is that it would error out if you tried to set a key that wasn't already explicitly set, even if a default was defined. This change fixes that to allow setting those. We still do validation on the "parent" keys when a setting path is drilling down into dicts, but any leaf key can now be set. Testing Done: Saw that I could now set items in siteconfig that I couldn't before.